Table 1. Average Within-Run Precision for ASAT, Uric Acid, and Spectrophotometric Absorbance of Reference Solutions with New Rotors and Rotors Washed One to Four Times

The Cobas-Bio Centrifugal Analyzer (Cobas; Roche Analytical Instruments, Nutley, NJ 07110) is currently used for various determinations, both by enzymatic and end-point techniques. A major advantage of the Cobas is its high-capacity (30 cuvettes) disposable rotor. Because the rotor is discarded after each run, cross-contamination between runs-which is a problem with centrifugal analyzers that have permanent rotors-is eliminated. However, this increases the expendables cost per test. This extra expense would be minimized if one could wash, dry, and reuse these rotors. We have constructed an apparatus for washing and drying Cobas rotors and have evaluated the reliability of the washed rotors for use in some laboratory tests. The washer, constructed mostly of polyvinyl chloride and operated by pressurized water, measures 71/2 X 71/2 X 4 in. A rotor is placed on a platform and spins at about one revolution per minute under the impact of the water, during which the individual cuvettes are repeatedly rinsed as they pass each water jet, flushing away the mixture of serum and reagents. A 5-mm timer is set to minimize water use. The dryer measures 9Y4 X 161/4 X 9 4 in. and is operated by pressurized air warmed to 40 #{176}C, which is sent into the individual cuvettes of the rotor. Up to five rotors can be dried simultaneously in 20 mm. Detailed plans of the washer/dryer are available upon request. Several clinical tests were used in the evaluation of the cleaning efficiency of the washer and dryer system, in which we compared washed and new rotors for within-run precision, day-to-day precision, and agreement between test results for patients’ specimens run on both new and washed rotors. The effectiveness of the washer was also evaluated for removing selected substances from the rotors, including serum proteins, substrates, and specific enzymes from specimens with high activities, as well as reagent carryover from other tests. Within-run precision for several tests was evaluated with new rotors as cornpared with the same tests run with washed rotors. The tests included aspartate aminotransferase (ASAT) (1), uric acid by an end-point technique (2), and the absorbance of various reference solutions. The precision of results from four new rotors was compared with results obtained with use of a single rotor washed one to four times on successive days. For ASAT and uric acid, each rotor was filled with 10 replicates at a low concentration and 10 replicates at a high concentration. For the absorbance measurements, 10 replicates were run on 0.2 mmol/L solutions of potassium dichromate, at 350 nm; 0.04 mmol/L 4nitrophenol, at 401 nm; and 74 mmol/L cobaltous ainmonium sulfate, at 512 nm. As shown in Table 1, no significance difference in average precision (F-test) was observed between new and washed rotors at the 95% confidence interval. These same data were used to determine if multiple washings led to progressive deterioration of rotor quality. As shown in Table 1, there was no statistically significant loss of precision for any testing in which rotors washed two, three, or four times were used as cornpared with the average precision for these tests when new rotors were used. In a more extensive study, we ran the alkaline phosphatase procedure (3) on a single rotor, used once and washed once each working day for three weeks. No deterioration in precision was observed. However, after repeated washing and handling, the rotors tended to break apart. Even though the pieces can be reassembled into the Cobas and reused, we discard them once they break. Thus it appears that reuse of rotors may be limited by physical strength more than by chemical contaminations. Results for ASAT and alanine aminotransferase (ALAT) (4) were compared for 120 different patients’ specimens for which we used new and washed rotors. Activities for ASAT and ALAT were all <440 U/L (reference interval 5-30 U/L for each). Linear least squares regression analysis yielded the following equations: