Beyond the standard semen analysis, there are specialized tests that answer specific questions: whether there is an infection, whether the sperm carry the right number of chromosomes and whether their DNA is intact. They are not done routinely, but for a specific indication.
Semen culture
A semen culture is a laboratory test performed to check for pathogenic micro-organisms in the semen and, if the result is positive, to choose the right antibiotic. The indications for it are sudden symptoms in the lower urogenital tract, such as painful ejaculation, haematospermia (blood in the semen) or difficulty passing urine, as well as a high number of white blood cells on a previous semen analysis. Before the test the patient must not have taken antibiotics for at least 7 to 10 days. Particular care must also be taken to avoid contaminating the sample, which is why collection takes place in a sterile container after washing the hands and the penis. The sample is then cultured on a non-selective medium that favours bacterial growth. If a bacterial strain is present it is identified, and after antibiotic sensitivity testing the doctor chooses the appropriate treatment.
Fluorescence in situ hybridization (FISH on sperm)
Fluorescence in situ hybridization is a technique used to examine the genetic material of cells. It relies on probe molecules that bind to particular chromosomal regions and fluoresce under light of the appropriate wavelength. Each probe binds to one chromosome only, and the fluorescent signal makes it possible to count the chromosomes.
Humans are a diploid species and so have two copies of each chromosome, one inherited from the father and one from the mother. In a normal spermatozoon, therefore, we expect the genetic material to be present in a single copy. Errors during gametogenesis can lead to sperm with the wrong number of chromosomes (aneuploid sperm), which can be detected by FISH on sperm. Some aneuploid sperm are normal, but when the proportion is high the result can be miscarriage, reduced implantation rates and an increased chance of children being born with an abnormal karyotype. In such cases, after reviewing the couple's history, preimplantation testing before embryo transfer may be suggested in order to select embryos with the right number of chromosomes.
The chromosomes most often tested are 13, 18, 21, X and Y. It is also possible to test the remaining chromosomes if the patient's medical history calls for it.
Sperm DNA fragmentation testing (DFI)
Damage to sperm DNA is any chemical change to its physical structure. DNA fragmentation is one such change and can be due to errors during spermatogenesis, or be related to processes such as cell death and oxidative stress. Exposure to toxic substances, high temperatures, smoking, chemotherapy and even increasing age can lead to a higher rate of fragmentation. The commonest test for assessing male fertility, the semen analysis, does not take fragmentation into account. A correlation has been observed between poor morphology and low motility and a high DFI, although a high DFI can also occur in normal samples. Although the fertilizing capacity of the sperm does not appear to be adversely affected, many studies show a negative correlation between DNA fragmentation and embryo development, implantation capacity and live births. The techniques used to determine DFI in semen differ considerably from one another. Because of these differences it is impossible to propose universal limits for normal and abnormal values; instead, each laboratory sets its own limits, depending on the technique and protocol it uses.
The test is recommended for
- Men with a varicocele.
- Couples with recurrent miscarriage.
- Couples in whom a previous IVF attempt showed poor development at the blastocyst stage.
- Men with an abnormal semen analysis.