Fructose makes up 99% of the reducing sugar present in semen. This sugar is produced in the seminal vesicles. Diminished levels of fructose have been shown to parallel androgen deficiency and the testosterone level. Following testosterone therapy, the level of fructose increases.
Although the fructose test is not part of a routine semen analysis, it is useful in cases of azoospermia (absence of sperm in semen). In azoospermia secondary to the absence of vesicles or if there is an obstruction, no fructose is present. In testicular azoospermia, fructose is present. When azoospermia and low semen volume exists, the fructose test should also be done, on a postejaculate urine sample to check for retrograde ejaculation. This occurs when the ejaculate goes into the bladder instead of out the urethra.
The procedure for determining the amount of fructose in semen involves heating semen in a strong acid in the presence of resorcinol. Fructose gives a red color (Selivonoff reaction) and may be read in a photometer. The normal average is 315mg/dL fructose.