His count was normal. So why didn't it work?

Sperm DNA fragmentation means breaks in the DNA packed inside the sperm head, and every man has some.

The question is whether there is an unusual amount. A sperm with damaged DNA can look normal, swim well and still fertilise an egg. That is why a routine semen analysis cannot find it, and why a man with a normal count can still have a high result.

He gives one semen sample, the same way as for a semen analysis, and the lab stains or treats the sperm to count how many carry DNA breaks.

The result is the DNA fragmentation index (DFI): the percentage of sperm showing damage. Several methods exist (SCSA, TUNEL, SCD or Halosperm, Comet). They measure related but different things, so each has its own threshold. The sperm tested are damaged by the test, so they are never the sperm used in treatment.

It is worth doing when a result would change the plan: after recurrent miscarriage, repeated cycles with poor embryos and no other cause, unexplained infertility after both partners are fully tested, or a borderline decision about treating a varicocele.

In each case something has already happened that needs explaining. At the start of a straightforward work-up, it rarely changes what we do next. Like any semen test, one raised result during or after an illness may not be his baseline, so it is repeated before anything irreversible is decided, usually about three months after a cause is corrected.

A DFI result cannot tell you whether a particular cycle will work, and a high result does not mean he cannot father a child.

Men with high results conceive, naturally and with treatment. A normal result does not rule out other causes. It is also not a verdict on how he has lived: DNA breaks rise with ordinary things like age, heat, long drives and a spell of heavy drinking.

A sperm aneuploidy test (sperm FISH) counts sperm carrying the wrong number of chromosomes; it answers a different question from DNA fragmentation and is rarely useful.

It is not part of routine guideline testing for male infertility. It may be discussed in rare cases, such as repeated pregnancies with a chromosome error or a known chromosome problem in the man, usually alongside karyotyping and genetic counselling. Testing the embryos (PGT) looks at the embryo itself, and is discussed only in selected cases.

We ask one question of every extra test: what will we do differently depending on the result?

If there is no clear answer, the test is not worth your money yet. The same rule made us stop recommending a test we had used for years.