Sperm DNA Fragmentation

Sperm DNA fragmentation testing measures the proportion of sperm whose DNA is broken. A routine semen analysis counts sperm, judges their movement and shape, but it does not show the integrity of the genetic material inside them. DNA damage in sperm is linked to lower fertilisation, poor embryo development, miscarriage and, in some studies, failure of assisted reproduction.

The damage arises in the testis (from faulty chromatin packaging and failed repair during spermatogenesis), and along the epididymis and ejaculate, where oxidative stress from white cells, infection, varicocele and heat breaks DNA strands. Causes include varicocele, genital tract infection, fever, smoking, obesity, heat exposure, older age, toxins, radiotherapy and chemotherapy, and long abstinence periods.

Several methods are used. The sperm chromatin structure assay uses flow cytometry and a dye that changes colour when DNA is damaged, and it reports the DNA fragmentation index. The TUNEL assay labels DNA breaks directly. The sperm chromatin dispersion (halo) test and the comet assay are other methods. Results are given as the percentage of sperm with fragmented DNA. The test takes 1 to 3 days in the laboratory.

Fragmentation is under about 15 to 20% in fertile men, and it is above 25 to 30% in about 20 to 30% of men with unexplained infertility. A level of over 25 to 30% is linked to lower rates of natural pregnancy and higher rates of miscarriage, and testicular sperm have lower fragmentation than ejaculated sperm in men with high levels.

Indications.

• Unexplained infertility with a normal semen analysis.
• Recurrent pregnancy loss (two or more miscarriages) with a normal female workup.
• Repeated failure of assisted reproduction (intrauterine insemination or in vitro fertilisation) or poor embryo quality.
• Varicocele, to help decide on repair, and men with risk factors (heat exposure, smoking, obesity, infection).
• Before assisted reproduction in men with a high risk profile, to decide whether to use testicular sperm.
• Men exposed to gonadotoxic treatment or toxins.

Not indicated as a routine first test in all infertile men, or when the semen analysis shows severe abnormalities that already direct treatment.

Specimen requirements.

• Fresh semen after 2 to 5 days of abstinence (a short, consistent period is important), collected by masturbation into a clean, wide mouthed container, and delivered within 1 hour, kept at body temperature.
• A routine semen analysis on the same sample, with count, motility and morphology.
• For some methods, the sample is frozen or fixed as the laboratory instructs.
• Clinical information: fever in the previous 3 months, infection, medicines, smoking, and varicocele.
• Consent and counselling about what the result can and cannot say.

Normal appearances.

The proportion of sperm with fragmented DNA is low. As a general rule, a DNA fragmentation index under 15 to 20% is regarded as good, 20 to 30% is intermediate, and over 30% is high, though each method has its own thresholds, and the laboratory report states them.

Interpretation.

• Low fragmentation: DNA damage is unlikely to explain the infertility, and other causes are sought in both partners.
• Intermediate fragmentation: a possible contributor, and risk factors are looked for.
• High fragmentation: linked to lower natural conception, lower intrauterine insemination success, higher miscarriage, and poorer embryo development. The chance of pregnancy with intracytoplasmic sperm injection is less affected than with conventional fertilisation.
• High values together with a varicocele, infection or a modifiable risk factor suggest that treatment of the cause may lower the level.
• Testicular sperm have less fragmentation than ejaculated sperm in men with high ejaculate values, which supports use of testicular sperm in selected cases.
• Results change over time, and a repeat test after a period of 3 months or after treatment gives better information.
• The result has to be read together with the semen analysis, female factors, and the age of the partner.

Critical values and findings needing immediate action.

• There are no emergency values.
• Very high fragmentation (over 50%) in a couple with repeated failure: urgent referral to a fertility specialist before further cycles.
• A finding suggesting infection (white cells in semen): culture and treatment.
• Marked abnormal results in a young man with cancer treatment planned: sperm preservation before treatment.

Limitations and artefacts.

• Methods differ, and thresholds are not the same, so values from different laboratories cannot be compared directly.
• Results depend on the abstinence time, fever, and the time between collection and testing. Sperm DNA damage rises if the sample is kept too long or at high temperatures.
• A single test varies from day to day, and the test is repeated when the result changes management.
• The clinical value is debated, since studies differ in their design and results, and no single threshold predicts outcome in an individual couple.
• The test cannot say which sperm fertilises the egg, nor whether the damage is repairable by the egg.
• Antioxidant treatment, varicocele repair, and other measures lower the level in some men, but the effect on live birth is uncertain.
• Cost and limited availability.

Special groups.

• Men with varicocele: the test helps to decide on repair when other parameters are borderline.
• Older men: fragmentation rises with age.
• Couples with recurrent miscarriage: both partners are assessed, including a female factor review.
• Men with cancer: high levels are possible after treatment, and sperm is banked before treatment where possible.
• Men with spinal injury or other causes of ejaculatory failure: fragmentation can be high in stored semen.
• Regions: the test is available in few centres. Basic measures that are known to help (treat infection, stop smoking, reduce heat exposure, control weight and diabetes, avoid long abstinence, and review medicines and toxins) are available everywhere and cost little. Varicocele is common in many regions, and diagnosis rests on examination and scrotal ultrasound.

Follow up tests.

• Repeat semen analysis, and repeat DNA fragmentation after about 3 months or after treatment.
• Semen culture, and tests for infection.
• Scrotal ultrasound for varicocele and other lesions.
• Hormone tests (follicle stimulating hormone, luteinising hormone, testosterone, prolactin).
• Karyotype and Y-chromosome microdeletion testing in severe oligozoospermia.
• Female partner assessment: ovarian reserve, tubal patency, uterine cavity and ovulation.
• Testicular sperm retrieval, when ejaculated sperm have very high fragmentation.