Abstract:
A method of separating a first class of lipoprotein in a sample from a second class of lipoprotein in the sample including: precipitating the second class of lipoprotein; contacting the sample with a magnetically responsive particle; and placing the sample in a magnetic field until the magnetically responsive particle has sedimented, thereby causing the precipitated second class of lipoproteins to sediment, leaving the first class of lipoproteins in the supernatant of the sample.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the fractionation of lipid mixtures. 
     Plasma lipoproteins are spherical particles which contain varying amounts of cholesterol, triglycerides, phospholipids, and proteins. They include an outer surface composed of phospholipid, free cholesterol, and protein, and an inner core containing mostly esterified cholesterol and triglycerides. Plasma lipoproteins serve to solubilize and transport cholesterol and triglyceride in the bloodstream. 
     The relative proportion of protein and lipid in a plasma lipoprotein determines the density of the plasma lipoprotein, Gotto, 1988, Hosp. Pract. 23:4 (Suppl. 1). Based on their density, particle size, composition, and electrophoretic mobility, circulating lipoproteins have been categorized into four major classes. The classes are: chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Some of the characteristics of these classes are shown in Table 1. 
     
                       TABLE 1______________________________________CHARACTERISTICS OF PLASMA LIPOPROTEINS  Diameter Density  (Angstroms)           (g/ml)    Origin______________________________________Chylomicrons      750-12,000               &lt;0.95     intestineVLDL     300-700    &lt;1.006    liverLDL      180-300    1.019-1.063                         catabolism of VLDLHDL       50-120    1.063-1.21                         liver &amp; intestine______________________________________ 
    
     The major classes of lipoproteins can be further divided into subclasses. LDL includes at least 7 subclasses, as is described in McNamara, 1990, AACC Lipids and Lipoproteins Division Newsletter 4:1, hereby incorporated by reference, Krauss et al., 1982, J. Lipid Res. 23:97, hereby incorporated by reference, and McNamara et al., 1987, Arteriosclerosis 7:483, hereby incorporated by reference. HDL includes at least two subclasses, HDL 2  and HDL 3 , as is described in Whitaker, 1986, Clin. Chem. 32:1274, hereby incorporated by reference and Anderson, 1977, Biochem. Biphys. Acta 493:55, hereby incorporated by reference. 
     The major risk factors for coronary heart disease are hypercholesterolemia, cigarette smoking, hypertension, diabetes, obesity, and male sex. Although hypercholesterolemia in general is the most prominent of these risk factors, numerous clinical studies have shown that the different lipoprotein classes have very distinct and varied effects on heart disease, Crouse et al., 1985, J. Lipid Res., 26:566-572; Kannel et al., 1979, Ann. Intern. Med., 90:85-91; Kannel et al., 1984, Circulation 70:157A-205A. In fact the presence of HDL provides a protective effect against coronary heart disease, and therefore relatively low HDL-cholesterol levels may be indicative of greater risk, Miller et al., 1975, Lancet 16:23; Castelli et al., 1977, Circulation 55:767; Gordon et al., 1977, Am. J. Med. 62:707; Heis et al., 1980 Circulation 62:116 (Suppl. 4). 
     Recognition of the importance of HDL cholesterol as a strong inverse risk factor for coronary artery disease has led to substantial demand for an HDL cholesterol assay suitable for clinical and research use. Various methods, including ultracentrifugation, electrophoresis, and specific precipitation, Havel et al., 1955, J. Clin. Invest. 34:1345, have been used to separate various classes of lipoproteins. 
     Precipitation-based methods have been used widely for routine quantitation of lipoproteins. Several precipitation-based methods have been described in the recent literature. Most of these methods are based on earlier work by Burstein and colleagues (reviewed in Burstein, M., and Scholnick, J. R., Lipoprotein-polyanion-metal interactions, In, Advances in Lipid Research 11, R. Paoletti and D. Kritchevsky, Eds., Academic Press, New York, N.Y. 1973, pp. 67-108). In these methods, the fractionation of lipoproteins in a solution, e.g., serum, is accomplished primarily by selective precipitation followed by centrifugation. The supernatant is then used to determine the cholesterol content remaining, or, by difference, the cholesterol content of that fraction which has been specifically removed from the supernatant. For example, low density lipoprotein (LDL) cholesterol, can be determined in this manner by specifically precipitating only LDL using heparin in citrate buffer at pH 5.04. Others have reported using polyvinyl sulfate or polyethylene glycol to effect specific precipitation of LDL and VLDL. In each case, centrifugation is necessary before measuring the cholesterol content. 
     Similarly, the measurement of HDL-cholesterol is usually a two-step process in which HDL is first separated from the other apoB-containing plasma lipoproteins, and the cholesterol content of the HDL-containing fraction measured. The most commonly employed methods are based on those developed by Burstein and his colleagues (Burstein et al., 1988, in Clarkson TB, Kritchevsky D, Pollak OJ (eds): Monographs on Artherosclerosis, New York, Karger) in which the apoB-containing lipoproteins are removed by precipitation with a polyanion in combination with a divalent cation, Bachorik et al., 1986, Methods Enzymol. 129:78. The two most commonly used precipitants are sodium phosphotungstate-MgCl 2  and dextran sulfate (m.wt 50,000)-MgCl 2 , Warnick et al., 1982, Clin. Chem. 23:1379. Other precipitants used include heparin sulfate-MnCl 2  and heparin sulfate-calcium carbonate. In these assays the precipitating reagent is added to an aliquot of serum or plasma. A heavy white precipitate forms immediately, and the mixture is allowed to stand until precipitation is complete. The precipitate is then sedimented by centrifugation, and an aliquot of the clear supernatant is removed for cholesterol analysis. 
     The first method used extensively in major population studies employed heparin and Mn ++  to precipitate VLDL and LDL, allowing HDL to be measured in terms of the amount of cholesterol remaining in the supernatant solution, Burstein et al., 1960, Clin. Chim. Acta 5:609, Frederickson et al., 1968, J. Clin. Invest. 47:2446-2457, Manual of Laboratory Operations, Lipid Research Clinics Program, Lipid and Lipoprotein Analysis, I. National Heart and Lung Institute. DHEW Publications No. (NIH) 75-628, 1974. This method has been extensively studied, Bachorik et al., 1976, Clin. Chem. 22:1828-1834, Warnick et al., 1978a, J. Lipid Res. 19:65-76, and modifications have been described, Warnick et al., 1978a, supra; Warnick et al., 1978b, Clin. Chem. 24:900. 
     SUMMARY OF THE INVENTION 
     In general, the invention features a method of separating a first class of lipoprotein, e.g., any of HDL, LDL, or VLDL, in a sample from a second class of lipoprotein, e.g., any or all of the remaining types of lipoproteins in the sample, e.g., separating HDL from LDL and/or VLDL, separating LDL from HDL and/or VLDL, or separating VLDL from LDL and/or HDL. The method includes: precipitating the second class of lipoprotein, preferably by contacting the sample with a precipitating reagent; contacting the sample with a magnetically responsive particle; and placing the sample in a magnetic field until the magnetically responsive particles have sedimented, thereby causing the precipitated second class of lipoproteins to sediment and leaving the first class of lipoproteins in the supernatant of the sample. 
     Preferred embodiments include those in which the precipitating reagent is contacted with the sample prior to contacting the sample with the magnetically responsive particles; the precipitating reagent and the magnetically responsive particles are contacted with the sample simultaneously, i.e., they are added simultaneously; the precipitating reagent includes e.g., dextran sulfate and MgCl 2 , or phosphotungstic acid and MgCl 2  ; the magnetically responsive particles include polyacrolein and a form of iron e.g., iron oxide; the sample contains up to 1,300 mg/dl triglycerides; the sample is a biological fluid, e.g., blood, plasma, or serum, from an animal, e.g, a vertebrate, e.g., a mammal, e.g., a human; the sample will not be returned to an animal; and the sample will be returned to an animal e.g., the animal from which the sample was taken. 
     In another aspect the invention features a method of measuring the amount of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, contained in a first class of lipoprotein, e.g., HDL, LDL, or VLDL in a sample. The sample includes a first and a second class of lipoprotein, the second class including any other class of lipoprotein, e.g., if the first class is HDL, the second class can be LDL, VLDL, or both. The method includes: precipitating the second class of lipoprotein, preferably by contacting the sample with a precipitating reagent; contacting the sample with a magnetically responsive particle; placing the sample in a magnetic field until the magnetically responsive particles have sedimented, thereby causing the precipitated second class of lipoproteins to sediment, leaving the first class of lipoproteins in the supernatant of the sample; and determining the amount of cholesterol in the first class of lipoprotein. 
     Preferred embodiments include those in which the amount of the constituent of interest in the first class is determined by determining the amount of the constituent in the supernatant, and those in which the amount of the constituent in the first class is determined by determining the total amount of the constituent present in the sample, determining the amount of the constituent in the sedimented second class, and subtracting the latter from the former. 
     Preferred embodiments include those in which: the precipitating reagent is contacted with the sample prior to contacting the sample with the magnetically responsive particles; the precipitating reagent and the magnetically responsive particles are contacted with the sample simultaneously, i.e., they are added simultaneously to the sample; the precipitating reagent includes e.g., dextran sulfate and MgCl 2 , or phosphotungstic acid and MgCl 2  ; the magnetically responsive particles include polyacrolein and a form of iron, e.g., iron oxide; the sample contain up to 1,300 mg/dl triglycerides; the sample is a biological fluid, e.g., blood, plasma, or serum, from an animal, e.g, a vertebrate, e.g., a mammal, e.g., a human; the measurement is performed on an automated device; and the measurement is performed on a manually operated spectrophotometer. 
     Preferred embodiments include those in which the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the HDL of a sample is determined by precipitating and sedimenting the LDL and VLDL in the sample, then measuring the concentration of the constituent in the supernatant; the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the LDL of a sample is determined by precipitating and sedimenting the HDL and VLDL in the sample, then measuring the concentration of the constituent in the supernatant; the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the VLDL of a sample is determined by precipitating and sedimenting the LDL and HDL in the sample, then measuring the concentration of the constituent in the supernatant; the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the HDL of a sample is determined by precipitating and sedimenting the LDL and VLDL in the sample, determining the amount of the constituent in the sediment, determining the total amount of the constituent in the sample, then subtracting the former from the latter; the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the LDL of a sample is determined by precipitating and sedimenting the HDL and VLDL in the sample, determining the amount of the constituent in the sediment, determining the total amount of the constituent in the sample, then subtracting the former from the latter; and the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the VLDL of a sample is determined by precipitating and sedimenting the HDL and LDL in the sample, determining the amount of the constituent in the sediment, determining the total amount of the constituent in the sample, then subtracting the former from the latter. 
     The invention also includes a method of measuring the amount of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, contained in a first class of lipoprotein, e.g., HDL, or LDL, or VLDL, in a sample. The sample includes a first and a second class of lipoprotein, wherein the second class of lipoprotein can include any other class of lipoprotein, e.g. if the first class is HDL, the second class can be LDL, or VLDL, or both. The method includes: precipitating the first class of lipoprotein, preferably by contacting the sample with a precipitating reagent; contacting the sample with magnetically responsive particles; placing the sample in a magnetic field until the magnetically responsive particles have sedimented, thereby causing the precipitated first class of lipoproteins to sediment, leaving the second class of lipoproteins in the supernatant of the sample; and determining the amount of cholesterol in the first class of lipoprotein. 
     Preferred embodiments include those in which the amount of the constituent of interest in the first class is determined by determining the amount of the constituent in the sedimented first class, and those in which the amount of the constituent in the first class is determined by determining the total amount of the constituent present in the sample, determining the amount of the constituent in the supernatant, and subtracting the latter from the former. 
     Preferred embodiments include those in which the precipitating reagent is contacted with the sample prior to contacting the sample with the magnetically responsive particle; the precipitating reagent and the magnetically responsive particles are contacted with the sample simultaneously, i.e., they are added to the sample simultaneously; the precipitating reagent includes e.g., dextran sulfate and MgCl 2 , polyethylene glycol, heparin and citrate, or phosphotungstic acid and MgCl 2  ; the magnetically responsive particles include polyacrolein and a form of iron, e.g., iron oxide; the sample contains up to 1,300 mg/dl triglycerides; the sample is a biological fluid, e.g., blood, plasma, or serum, from an animal, e.g, a vertebrate, e.g., a mammal, e.g., a human; the measurement is performed on an automated device; and the measurement is performed on a manually operated spectrophotometer. 
     Preferred embodiments include those in which the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the LDL of a sample is determined by precipitating and sedimenting the LDL in the sample, then measuring the level of the constituent in the sediment; the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the VLDL of a sample is determined by precipitating and sedimenting the VLDL in the sample, then measuring the level of the constituent in the sediment; the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the HDL of a sample is determined by precipitating and sedimenting the HDL in the sample, then measuring the level of the constituent in the sediment; the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the LDL of a sample is determined by precipitating and sedimenting the LDL in the sample (leaving the HDL and VLDL in the supernatant), determining the amount of the constituent in the supernatant, determining the total amount of the constituent in the sample, then subtracting the former from the latter; the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the HDL of a sample is determined by precipitating and sedimenting the HDL in the sample, (leaving the LDL and VLDL in the supernatant) determining the amount of the constituent in the supernatant, determining the total amount of the constituent in the sample, then subtracting the former from the latter; and the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the VLDL of a sample is determined by precipitating and sedimenting the VLDL (leaving the LDL and HDL in the supernatant) in the sample, determining the amount of the constituent in the supernatant, determining the total amount of the constituent in the sample, then subtracting the former from the latter. 
     A class of lipoprotein, as used herein, can refer to one of the major classes of lipoprotein, e.g., HDL, LDL, or VLDL, or one of the subclasses of the major classes, e.g., one of the seven subclasses of the LDL major class or one of the two subclasses of the HDL major class. 
     Methods of the invention use magnetically responsive particles for the rapid removal of lipoproteins that have been selectively precipitated from an aqueous medium such as blood, plasma, or serum. Preferred embodiments provide enhanced fractionation by selective precipitation of lipoproteins of one or more classes (e.g., any of HDL, LDL or VLDL) coupled with magnetically induced sedimentation. Fractionation is an important step in the measurement of pathologically significant lipid components such as HDL cholesterol, LDL cholesterol, and VLDL cholesterol. 
     The use of magnetizable particles eliminates or reduces the need for a centrifugation step in plasma lipoprotein fractionation. As a result, magnetically based separations are relatively rapid, do not require expensive or energy consuming equipment, and reduce radiological, biological, and physical hazards. Furthermore, samples are not exposed to centrifuge-generated heat, which can compromise the integrity of the samples. 
     Methods of the invention allow for complete sedimentation of precipitated lipoproteins without additional dilution of the sample, even in samples with high concentrations of triglycerides. Thus, the time and expense required for diluting samples, and errors in estimation that arise from dilution, are eliminated. 
     Magnetic separation methods of the invention can be incorporated into an automated clinical analyzer (of the type commonly used for cholesterol measurements) to allow in-line separation and direct automated measurement of HDL-cholesterol, and/or LDL-cholesterol, and/or VLDL-cholesterol. 
     The magnetic separation methods of the invention can be used in the measurement of any lipoprotein component, e.g., phospholipids, triglycerides, apolipoproteins, or cholesterol esters, found in a class of lipoprotein. 
     Other features and advantages of the invention will be apparent from the following description of the preferred embodiments, and from the claims. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     REAGENTS 
     A &#34;magnetically responsive particle&#34; or &#34;magnetic particle&#34;, as used herein, is any particle dispersable or suspendable in aqueous media and separable from suspension by application of a magnetic field. This includes particles that are intrinsically magnetically responsive or particles that have been rendered magnetically responsive, e.g., by attachment to a magnetically responsive substance or by incorporation of such substance into the particles. The magnetic component can be incorporated into a particle e.g., by impregnating the substance in a polymeric matrix such as cellulose or polyacrolein. The magnetic particles can be ferromagnetic, superparamagnetic, or more preferably paramagnetic. 
     The magnetic component of magnetically responsive particles can be any component that is susceptible to a magnetic field, e.g., salts, oxides, borides, or sulfides of iron, cobalt, or nickel; rare earth elements having high magnetic susceptibility, e.g., hematite or ferrite; or pure magnetically responsive metals or alloys thereof. A wide variety of particles e.g., cellulose/iron oxide or polyacrolein/iron oxide, are suitable for use in the invention. 
     Magnetic particles can be obtained from Cortex Biochem., Inc., (San Leandro, Calif.). Several types of magnetic particles were tested. They are, in order of descending preference, polyacrolein/iron oxide (e.g., Cortex #CM1001), cellulose/iron oxide (e.g., Cortex #CM1000), low density cellulose/iron oxide (e.g., Cortex #CM1003), and magnetizable charcoal (e.g., Cortex #CM5002). 
     Methods for the fabrication of magnetically responsive particles are known to those skilled in the art, see e.g., U.S. Pat. No. 4,628,037, hereby incorporated by reference, which discloses the production of particles with magnetic metal oxide cores; U.S. Pat. No. 4,687,748, hereby incorporated by reference, which discloses the preparation of spheres composed of magnetic particles included in a polymer matrix, e.g., in a carbohydrate matrix; U.S. Pat. No. 4,795,698, hereby incorporated by reference, which discloses the production of magneticpolymer particles by the coprecipitation of transition metals in the presence of a polymer having available coordination sites; and U.S. Pat. No. 4,661,408, hereby incorporated by reference, which discloses the preparation of magnetically responsive particles with a core of CrO 2 . Magnetically responsive particles are also commercially available, as described herein. 
     The magnetically responsive particles may be added sequentially or simultaneously with the precipitating reagent. If added simultaneously, (using a single precipitation/magnetically responsive particle reagent) the size and surface characteristics should be chosen to allow the particles to remain in suspension until the precipitation is complete--at which time magnetic separation is applied. For this type of &#34;simultaneous addition&#34; procedure, particles in the range of 1-10 microns have been used for selective lipoprotein precipitation assays. 
     Magnetically responsive particles smaller than 1 micron will also work well since they are likely to remain in suspension until the magnetic field is applied. The upper limit of particle size depends on factors which, in addition to size, determine sedimentation rate in the absence of a magnetic field e.g., particle density, the nature of the functional groups on the surface of the particle (e.g., hydrophilic groups would result in relatively slower sedimentation while hydrophobic groups would result in more rapid sedimentation), and on the rate of reaction for the specific lipoprotein precipitation reaction being used. The particles must remain in suspension long enough to allow precipitation of the desired lipoprotein. 
     Factors affecting the choice of size of a magnetically responsive particle are known to those skilled in the art, see e.g., European Patent Application No. 86309967.7, hereby incorporated by reference; U.S. Pat. No. 4,628,037, hereby incorporated by reference; U.S. Pat. No. 4,687,748, hereby incorporated by reference. 
     The reagent used for the specific lipoprotein precipitation will vary with the lipoprotein to be precipitated and can be chosen by methods known to those skilled in the art. Heparin and citrate, heparin-Mn ++ , dextran sulfate-Mg ++ , polyethyleneglycol, polyethylene glycol-polyvinyl sulfate, and phosphotungstate-Mg ++  have all been used successfully for selective precipitation of one or more class of lipoproteins. 
     Precipitating reagents useful in the analysis of HDL-cholesterol are known to those skilled in the art and include the following: heparin/Mn ++  (available e.g., from WAKO Chemicals, Dallas, Tex.) (Burstein et al., 1960, supra, Warnick et al., 1978, J. Lipid Res. 19:65, hereby incorporated by reference); phosphotungstic acid/MgCl 2  (available e.g., from Sigma Chemical or Roche Diagnostics) (Drager et al., 1982 Lab Med. 6:198, hereby incorporated by reference, Burstein et al., 1969, Life Sci. 8:345, hereby incorporated by reference); dextran-SO 4  /MgCl 2  (available e.g, from DMA) (Warnick et al , 1982, Clin. Chem. 28:1379, hereby incorporated by reference); heparin/MgCl 2  /sucrose; (Burstein, 1962, C.R. Acad. Sci. 225:605, hereby incorporated by reference); Heparin/Ca ++  (Noma et al., 1978, Clin. Chem. 24:150, hereby incorporated by reference); heparin/Ca ++  /Ni ++  (Noma et al., 1979, Clin. Chem. 25:1480, hereby incorporated by reference); or polyethylene glycol, (available e.g., from Diagnostic Chemicals Ltd., Monroe, Conn.) (Vikari, 1976, J. Clin. Lab. Invest. 36:265 hereby incorporated by reference). 
     Subclasses of HDL can be separated from one another and from other classes of lipoprotein, see e.g., Whitaker et al., 1986, Clin. Chem. 32:1274, hereby incorporated by reference, or Gidez et al., 1982, J. Lipid. Res. 23:1206, hereby incorporated by reference. 
     Precipitating reagents useful in the analysis of LDL-cholesterol are known to those skilled in the art and include the following: heparin and citrate (available from Genzyme, One Kendall Square, Cambridge, Mass. or E. Merck, A.G. (Germany) as LDL cholesterin Cat. No. 14992) (Wieland et al., 1983, J. Lipid Res. 24:904, hereby incorporated by reference); polyvinylsulfate, (available from Boehringer, Mannheim (FRG) as LDL-cholesterol Cat. No. 726290) (Assmann et al., 1984, Clin. Chem. Acta 140:77,  hereby incorporated by reference, Maier et al., 1983, Clin. Chem. 29:1173, hereby incorporated by reference, Kerscher et al., 1985, Clin. Biochem. 18:118, hereby incorporated by reference); PVS/PEGME (polyvinyl sulfate polyethyleneglycol methyl ether) (available from BioMerieux, Cat. No. 61532, 69260 Charbonnieres, France) (Wehmeyer et al., Abstract Presented at 1983 national meeting of Am. Assoc. for Clin Chemistry, Boehringer Mannheim, GmbH, Research Center, Tutzing, Germany); heparin/Ca ++  /EDTA/lipase (Bartl et al., 1983, Clin. Chem. Acta 128:199, hereby incorporated by reference); dextran So 4  /Ca ++  (available from Immuno A.G. Vienna, Austria as Quantolip LDL-cholesterol) (Walton et al., 1964, J. Clin. Path. 17:627, hereby incorporated by reference, Cornwell et al., 1961, J. Lipid Res. 2:110, hereby incorporated by reference); heparin/resin (Noma et al., 1978, Clin. Chem. 24:1504, hereby incorporated by reference). 
     For each specific precipitation reagent one or more types of magnetically responsive particle can be applied over a wide range of particle concentrations, e.g., phosphotungstate or dextran sulfate can be used to precipitate LDL and VLDL, leaving HDL in the supernatant solution. With either of these reagent systems many magnetically responsive particles work well in a &#34;sequential addition&#34; mode, i.e., wherein the precipitating reagent is added first and the magnetically responsive particles are added after allowing precipitation to take place. Several also perform satisfactorily in the &#34;simultaneous addition&#34; mode, i.e., wherein the precipitating reagent and the magnetic particles are added simultaneously. Reagents, however, may be added in any sequence which results in effective precipitation and sedimentation. 
     The concentration of magnetically responsive particle needed to effect rapid complete separation will vary, e.g., with the concentration of lipid in the sample. The concentration for a given application can be determined by methods known to those skilled in the art and as described herein. In many cases the concentration of magnetically responsive particles in the reagent mixture will vary from 5 to 50 mg/ml and more preferably the concentration will vary from 15-25 mg/ml. 
     The precipitating reagent can be bound, by methods known to those skilled in the art, to the surface of the magnetically responsive particles, to enhance stability, lot-to-lot consistency, or to assure even faster separation, but the use of magnetically responsive particles to provide rapid, efficient separation usually does not require that the precipitating reagent be bound to the magnetic particles. 
    
    
     EXAMPLE 1: COMPARISON OF MAGNETIC PARTICLE-BASED SEDIMENTATION WITH A CENTRIFUGE-BASED METHOD FOR THE DETERMINATION OF HDL CHOLESTEROL CONTENT OF SERUM 
     In this example magnetic particle-based separation methods of the invention were compared with a method in which centrifugation was used to sediment the precipitated lipoproteins. In all cases lipoproteins were precipitated with dextran SO 4  /MgCl 2 . 
     Magnetic particles and precipitating reagent were combined to form a combined magnetic particle/precipitating reagent. Magnetic particles were added to the combined reagent as a 50 mg/ml slurry of particles in water. 100 ml of combined magnetic particle precipitating reagent included between 10 and 50 ml of slurry, 50 mls. of dextran sulfate-MgCl 2 , and water (if needed) to make 100 ml. The dextran sulfate-MgCl 2  solution was purchased from DMA or was formulated according to the method of Warnick et al., 1982, Clin. Chem. 28:1374, hereby incorporated by reference, from commercially available material. The data in Table 2 was obtained with dextran sulfate-MgCl 2  obtained form DMA. In the magnetic separations shown in Table 2, 0.1 mls of the combined precipitation reagent/magnetic particle reagent (50 ml magnetic particle slurry/100 ml combined reagent) was added to a 0.50 ml serum sample and allowed to incubate for 10 min. (Later experiments show that 1 min. of incubation is sufficient.) Samples to be magnetically sedimented were placed in a Serono Diagnostics magnetic rack for 1-10 min. (Usually 1-3 min. is sufficient.) 
     Magnetic particles were obtained from Cortex Biochem., Inc. (San Leandro, Calif.). Four types of particles were tested: M-1, M-2, M-3, and M-4. M-1 refers to Low Density Cellulose/iron oxide (#CM1004) (1-10μ in diameter); M-2 refers to Cellulose/iron oxide (#CM1000) (1-10μ in diameter); M-3 refers to Polyacrolein/iron oxide (CM1001) (1-10μ in diameter); M-4 refers to Magnetizable charcoal (#CM5002) (1-25μ in diameter). 
     For centrifuged samples, 0.05 ml of precipitating reagent was added to a 0.50 ml sample, allowed to stand for 10 minutes and then spun at 2000 rpm for 15 min. in a standard laboratory centrifuge. Total cholesterol in the supernatants (which is equivalent to HDL cholesterol) was analyzed by standard methods, and results expressed as mg/dl of cholesterol. 
     The results are shown in Table 2. 
     
                       TABLE 2______________________________________METHOD OF SEPARATIONCentrifugal    Magnetic SeparationPatient No.   Separation M-1     M-2    M-3    M-4______________________________________1.      39, 38     38, 39  39, 40 39, 39 44, 482.      42, 43     41, 41  40, 41 42.5, 42.5                                    N.G.3.      63, 59     58, 58  57, 58 55, 57 N.G.4.      43, 43     --      45, 46 46, 47 --5.      31, 31     --      34, 34 32, 34 --6.      34, 34     --      36, 37 36, 36 --7.      36, 37     --      37, 37 36, 37 --8.      45, 49     --      48, 48 47, 49 --9.      46, 47     --      47, 49 47, 50 --10.     51, 52     --      50, 51 51, 51 --11.     47, 48     --      44, 46 44, 44 --12.     43, 43     --      43, 44 45, 45 --13.     44, 44     --      44, 45 44, 46 --14.     51, 52     --      53, 55 53, 55 --15.     35, 35     --      35, 35 36, 36 --16.     41, 42     --      48, 48 40, 41 --17.     42, 42     --      34, 34 41, 43 --18.     28, 28     --      26, 29 31, 32 --19.     62, 62     --      60, 60 59, 60 --20.     48, 48     --      38, 39 47, 48 --21.     51, 51     --      51, 59 53, 51 --22.     33, 33     --      34, 34 33, 33 --______________________________________ 
    
     EXAMPLE 2: AMOUNT OF MAGNETIC PARTICLES REQUIRED. 
     The amount of magnetic particles needed for reliable results was determined by comparing the HDL-cholesterol values obtained in magnetic separation using differing amounts of polyacrolein/iron oxide particles. The amount of particles was varied by varying the amount of slurry added to 100 ml of combined reagent, as described above. 0.10 ml of combined reagent was added to 0.50 ml of sample. All other materials and methods are described in Example 1. The results are shown in Tables 3 and 4. The results are expressed as mg/dl HDL cholesterol. 
     
                       TABLE 3______________________________________METHOD OF SEPARATION             Magnetic Separations             Volume of slurryPatient  Centrifugal             (as % of combined reagent volume)No.    Separations             50%     40%   30%   20%   10%______________________________________1      69         74      67    68    68    --2      72         73      70    72    70    --3      53         54      49    50    51    --4      48         50      48    47    48    --5      44         48      43    42    44    --6      41         40      40    37    42    --7      44         42      43    44    46    --8      96         91      92    91    96    --9      43         40      52    43    43    --10     41         39      40    42    42    --11     36         33      35    36    36    --12     30         28      --    32    32    --13     57         54      57    56    58    --14     37         33      38    35    37    --15     85         80      83    82    84    --16     82         78      80    79    82    --17     51         49      --    51    51    --18     50         49      50    54    52    --19     48         46      46    49    49    --20     51         47      50    49    50    --21     38         38      37    40    38    --22     51         49      50    40    52    --23     50         49      48    --    52    --______________________________________ 
    
     
                       TABLE 4______________________________________METHOD OF SEPARATION               Magnetic Separations               Volume of slurryPatient Centrifugal (as % of combined reagent volume)No.     Separations 15%     17.5%  20%   22.5%______________________________________1       23, 24      23      24     24    232       40, 40      37      39     38    393       34, 30      31      32     32    334       40, 38      37      42     37    385       22, 21      21      22     22    216       58, 59      57      57     58    577       74, 75      72      75     76    748       10, 11      10      12     11    11______________________________________ 
    
     EXAMPLE 3: FURTHER COMPARISON STUDIES 
     In this example a magnetically based separation method was compared with a centrifugation based method. In Table 5, magnetic separation was performed with dextran sulfate/MgCl 2  (obtained from DMA) as the precipitating reagent (RDI HDL-M). In magnetically sedimented samples, 0.10 ml combined magnetic particle/precipitating reagent (20 ml magnetic particle slurry/100 ml combined reagent) was added to 0.50 ml of sample and the magnetic field applied 10 minutes later. Total sample cholesterol and total sample triglycerides were determined by standard methods and are shown in mg/dl/ All other methods and materials are as described in Example 1. 
     
                       TABLE 5______________________________________          METHOD OF SEPARATION                       Centrifugal                               Magnetic                       Separation                               SeparationPatient Total     Tri-        DMA     RDINo.   Cholesterol           glyceride   HDL     HDL-M______________________________________1     196       389         32      352     199       --          35      343     163       --          39      374     --        --          40      365     216       --          38      356     247       745         32      337     229       --          44      428     156       --          47      459     --        --          42      3810    245       --          65      6411    248        79         56      5412    226       209         38      3613    --        --          --      5014    289       157         78      7415    257       214         39      3816    273        64         54      5517    212       135         36      3518    238       201         39      3919    207        91         63      6220    223       195         48      4621    265       204         50      4822    175       --          46      4523    239       107         51      5024    236       157         47      4525    287       206         49      4726    215       296         29      2927    206        86         56      5328    217       330         44      4129    197       184         36      3330    254       130         51      4731    377       1,348       20      3132    193       --          59      5733     97       --          34      3234    156       --          19      1935    125       --          57      5836    204       --          37      3837    270       --          38      3938    201       --          58      5939    203       --          64      6540    240       --          37      3841    297       348         47      4842    261       --          33      3543    243       --          57      5744    103       --          45      4545    167       --          28      2946    180       --          58      5747    208       386         36      2948     99       --          47      4649    280       --          71      6850    156       --          44      4451    328       --          39      3852    216       --          70      7153    --                    35      3554    213       --          46      4755    205       --          45      4456    200       --          52      5157    169       --          44      4358    312       --          47      4659    242       --          47      47______________________________________ 
    
     The experiments described in Table 6 are similar to those described in Table 5 except that the dextran sulfate precipitating reagent used in the magnetically based method was not purchased from DMA but was formulated from commercially available dextran sulfate, as described above. 
     
                       TABLE 6______________________________________          METHOD OF SEPARATION                       Centrifugal                               Magnetic                       Separation                               SeparationPatient Total     Tri-        DMA     RDINo.   Cholesterol           glyceride   HDL     HDL-M______________________________________1     251       176         42      422     246        54         69      673     276       110         55      554     201       120         41      405     234       141         37      366     269       321         43      437     213       379         34      348     180        66         51      489     268        78         56      5410    252       115         53      5111    254       138         63      6012    217        89         62      6013    208       140         48      4914    185       103         35      3415    245       323         39      3716    138        77         34      3617    245       113         50      5218    268       143         56      5419    193       277         39      3620    177        79         47      3921    --        --          40      3922    191        90         56      5323    209       335         35      3124    231        62         79      7725    197        55         68      6526    191        90         55      5227    205       335         33      3128    185       117         50      4829    252       117         57      5430    220        90         51      4831    266        90         54      5432    240       157         52      5133    198       397         36      3734    260       118         56      5535    102        96         45      4136    202       159         56      5537    178        79         41      4238    171       121         42      4139    284        96         86      8440    298       169         49      4941    167        90         63      6142    142       104         25       843    167       118         32      3244    136       108         32      3345    196        82         58      5946     94        40         38      3947    157       177         31      3148    160        75         47      4949    108        61         34      3550    149        70         30      3251    217       216         40      3952    152        89         43      4253    212       167         52      5154    234       239         44      6755    145       100         26      2856    184       152         55      5057    105        86         31      3158    208       148         64      6159    147        99         29      3060    104        71         47      4861    272       159         57      5962    247       194         54      5563    251       167         59      6064    347       742         34      3465    274       242         39      4066    168        66         76      7467    286       275         31      3168    212       123         35      3669    190       134         50      5070    271       343         35      3571    233       380         34      3472    271       182         58      5773    222       134         31      3074    252       271         35      3475    240       285         34      3376    298       217         37      3777    320       254         30      3178    186       158         36      3479    232       108         59      5680    278       127         46      4481    281       136         49      4982    240        62         56      5583    231       124         50      4784    281        80         64      6085    269       269         63      5586    214       112         42      3987    251       109         51      48______________________________________ 
    
     Table 7 compares the effect of waiting 1 minute or 10 minutes after addition of the combined magnetic particle/precipitating reagent before applying the magnetic field. Other conditions were as described for Table 6. 
     
                       TABLE 7______________________________________          METHOD OF SEPARATION                  Magnetic          Centrifugal                  Separation                    Separation                            RDI    RDIPatient Total     Tri-     DMA     HDL-M  HDL-MNo.   Cholesterol           glyceride                    HDL     (10 min)                                   (1 min)______________________________________ 88   283       124      52      51     52 89   215        88      42      42     44 90   221       191      34      34     35 91   254       457      42      42     41 92   259       164      45      46     47 93   284       152      40      39     40 94   225       155      45      44     46 95   276       343      31      31     31 96   228       277      37      36     37 97   207       202      38      37     39 98   256       285      48      46     48 99   209       264      36      35     36100   209       503      46      45     42101   169       175      28      27     30102   119       100      30      31     32103   124       101      22      21     23104   118       105      26      26     27105    93        53      37      39     40106   111        54      38      38     40107   194       203      33      32     34108   231       298      57      45     46109   205       118      75      73     75110   184       223      31      32     33111   127       106      32      32     33112   158        73      41      41     41113   208       222      33      32     32114   273       326      26      25     --115   149        65      23      24     23116   239       132      35      34     36117   247       154      47      49     47118   266       136      39      38     38119   215       182      46      44     46120   268       174      43      42     43______________________________________ 
    
     EXAMPLE 4: MAGNETICALLY RESPONSIVE PARTICLE-BASED CLINICAL ASSAY FOR HDL CHOLESTEROL 
     The precipitating reagents dextran sulfate and MgCl, together, precipitate LDL and VLDL in serum. In this assay magnetically responsive particles, preferably polyacrolein-iron particles (polyacrolein:iron oxide (Fe 3  O 4 )=40:60) 1-10μ in diameter and the precipitating reagents are added to the sample simultaneously. After precipitation, the LDL and VLDL are pelleted by the application of a magnetic field to the sample HDL remains in the supernatant. The amount of HDL cholesterol is then assayed using an enzymatic reagent that measures total cholesterol. The intensity of color produced in the reaction is proportional to the concentration of HDL cholesterol. The assay is described in detail below. 
     The preferred sample is serum, though EDTA plasma may be used. The sample need not be fasting. Plasma (or serum) should be separated from the erythrocytes as soon as possible after collection since various changes can occur during storage. HDL cholesterol in plasma samples is stable for at least four days at 4°-8° C. or up to 2 weeks at -20° C. 
     Precipitation and fractionation are performed as follows: 
     a. Dispense 0.50 mL of each serum sample and control into an appropriately labeled test tube; 
     b. Add 0.10 mL of combined magnetic particles/precipitating reagent (dextran sulfate (0.1 mmol/l)), MgCl 2  (250 mmol/l), and magnetically responsive particles (10g/l) to the sample and vortex immediately for 10 seconds; 
     c. Incubate 1-10 minutes at 15°-30° C.; 
     d. Place the tubes on a magnetic surface and wait approximately 3 minutes for complete sedimentation of the magnetically responsive particles. Longer sedimentation times may be necessary if the sample has a high level of triglycerides (see below). In any case, the sedimentation time can be established by methods known to those skilled in the art. The assays are performed in 10×75 mm or 12×75 mm round bottom test tubes. Round bottom tubes are preferable to tubes with pointed or tapered bottoms because round bottom tubes result in a larger proportion of the sample being held more closely to the source of magnetism. The tubes are placed in racks that contain magnets in the base of the rack. Magnetic racks suitable for use in methods of the invention are available through: Serono Diagnostics, Allentown, Pa.; Gen-Probe, Inc., San Diego, Calif.; Ciba-Corning Diagnostics, E. Walpole, Mass.; Advanced Magnetics, Inc., Cambridge, Mass.; and Amersham Corp., Arlington Hts., Ill. A suitable rack exerts a magnetic flux density of approximately 175 to 265 gauss 0.5 inch from the surface upon which the bottom of the sample tube rests. 
     e. Obtain an aliquot of clear supernatant for the cholesterol assay by transferring the supernatant solution to a second labeled test tube for analysis. HDL cholesterol in the supernatant is stable for at least 72 hours when stored at 2°-8° C. 
     f. Determine cholesterol content with a cholesterol identifying agent, e.g., with the DMA Enzymatic Cholesterol Reagent Set (DMA Cat. No. 2340). Measurements can be converted to cholesterol concentration by comparison to known calibrators, using e.g., the DMA HDL cholesterol standard (DMA Cat. No. 2331-153). 
     g. Due to dilution of the sample during the precipitation step, multiply the HDL cholesterol concentration by 1.2 to obtain the final result. 
     Expected values for HDL are typically in the range of 30-70 mg/dL in males and 30-85 mg/dL in females, Each laboratory, however, should establish its own range of expected values. 
     DETERMINATION OF HDL-CHOLESTEROL IN SAMPLES WITH HIGH LEVELS OF TRIGLYCERIDES 
     Many precipitating reagents and sedimentation methods are not suitable for use with samples which contain high levels of triglycerides (greater than about 300-500 mg/dl) when centrifugation is used for sedimentation in that the supernatant is cloudy or turbid after precipitation and centrifugation. In these cases, samples with high levels of triglycerides must be diluted prior to precipitation to avoid erroneous cholesterol determinations. Methods of the invention, however, can be used on samples with triglyceride levels as high as 1000-1300 mg/dl, without dilution of the samples, although high triglyceride samples may require slightly longer sedimentation times than are used with normal samples. 
     Other embodiments are within the claims.