Semen analysis
A semen analysis evaluates certain characteristics of a man's semen and the sperm contained in the semen.  It may be done while investigating a couple's infertility or after a vasectomy to verify that the procedure was successful.

# Relation to fertility
The characteristics measured by semen analysis are only some of the factors in semen quality.  One source states that 30% of men with a normal semen analysis actually have abnormal sperm function.  Conversely, men with poor semen analysis results may go on to father children.

# Collection methods
The most common way to collect a semen sample is through masturbation, directing the sample into a clean cup.
A sample may also be collected during intercourse in a special type of condom.  Collection condoms are made from silicone or polyurethane, as latex is somewhat harmful to sperm.
The Catholic Church teaches that masturbation is immoral. For observant Catholics, collection condoms are the only morally permissible way to obtain semen samples. Catholics may put two or three pinholes in the collection condom to avoid violating the Catholic prohibition on contraception.
A third option for collecting a sample is through coitus interruptus (withdrawal).  With this technique, the man removes his penis from the woman near the end of intercourse and ejaculates into a cup.  This technique is mostly only recommended for men testing for success of a vasectomy, in contrast to an investigation of infertility, since the caregiver might assume that the meaning of the operation is for better quality of coitus for the man.
Finally, if a blockage in the vas deferens is suspected to impede fertility, semen can be taken directly from the epididymis. Such a sollection is called per cutaneous epididymal sperm aspiration (PESA). Alternatively, the testicular tissue itself, instead of the sperm produced can be investigated. Then, the collecting method is called TESE. 

# Tested characteristics
Examples of parameters measured in a semen analysis are: sperm count, motility, morphology, volume, fructose level and pH.

## Sperm count
Sperm count measures how many sperm are in a man's ejaculate.  Anything over 20 million sperm per milliliter is considered normal.  A vasectomy is considered successful if the sample is azoospermic, or if only rare non-motile sperm are observed (fewer than 100,000 per milliliter).

## Motility
The motility of the sperm is evaluated.  WebMD, a popular source of medical information on the internet, defines normal motility as 60% of observed sperm, or at least 8 million per milliliter, showing good forward movement. The World Health Organization (WHO) criteria are that normal motility is when at least 50% of observed sperm move forward normally.

## Morphology
The morphology of the sperm is also evaluated.  With WHO criteria, a sample is normal if 30% or more of the observed sperm have normal morphology.  If morphology is evaluated using the strict criteria developed by Dr. Thinus Kruger, a sample is normal if 14% or more of the observed sperm have normal morphology.

## Volume
The volume of the sample is measured.  WebMD advises that volumes between 1.0 mL and 6.5 mL are normal; WHO criteria specify that any volume greater than 2.0 mL is normal.  Low volume may indicate partial or complete blockage of the seminal vesicles, or that the man was born without seminal vesicles.

## Fructose level
The level of fructose in the semen is measured.  WebMD lists normal as at least 300 mg/mL.  WHO specifies a normal level of 13 mol per sample.  Absence of fructose may indicate a problem with the seminal vesicles.

## pH
The pH of the sample is measured.  WebMD lists a normal range of 7.1-8.0; WHO criteria specify normal as 7.2-7.8.  Acidic ejaculate (lower pH value) may indicate one or both of the seminal vesicles are blocked.  A basic ejaculate (higher pH value) may indicate an infection.  A pH value outside of the normal range is harmful to sperm.

## Others
The sample is tested for white blood cells.  A high level of white blood cells (over 1 million per milliliter) may indicate an infection.
The liquefaction time of the sample is measured. It normally takes less than 60 minutes for the sample to change from a thick gel into a liquid.  An abnormally long liquefaction time may indicate an infection.

# Factors that influence results
Compared to samples obtained from masturbation, semen samples from collection condoms have higher total sperm counts, sperm motility, and percentage of sperm with normal morphology. For this reason, they are believed to give more accurate results when used for semen analysis.
How long the man has abstained prior to providing the sample for analysis affects the results.  Longer periods of abstinence correlate with poorer results - one study found that men with repeated normal results produced abnormal samples if they abstained for more than 10 days.  It is recommended not to abstain for more than one or two days before providing the semen sample for analysis.
If the results from a man's first sample are subfertile, they must be verified with at least two more analysis.  At least 2 to 4 weeks must be allowed between each analysis.  Results for a single man may have a large amount of natural variation over time, meaning a single sample may not be representative of a man's average semen characteristics.  In addition, sperm physiologist Joanna Ellington believes that the stress of producing an ejaculate sample for examination, often in an unfamiliar setting and without any lubrication (most lubricants are somewhat harmful to sperm), may explain why mens' first samples often show poor results while later samples show normal results.
A man may prefer to produce his sample at home rather than at the clinic.  The site of semen collection does not affect the results of a semen analysis.

# Measurement methods
Computer Assisted Semen Analysis (CASA) is a catch-all phrase for automatic or semi-automatic semen analysis techniques. Most systems are based on image analysis, but alternative methods exist such as tracking cell movement on a digitizing tablet. Computer-assisted techniques are most-often used for the assessment of sperm concentration and mobility characteristics, such as velocity and linear velocity.