Recognition, prevalence and prevention of falsely high serum testosterone concentrations in testosterone gel users

Contamination of blood samples by application of testosterone gel near the venipuncture site may be a cause of falsely elevated serum testosterone (T) concentrations. We hypothesized that measuring estradiol (E) and calculating the T/E ratio could give a biochemical clue for contamination because skin-originating testosterone will not be aromatized to estradiol. We assessed the prevalence of abnormal T/E ratio in gel users and tested our hypothesis in 10 volunteers.

Retrospective study involving 277 transmasculine persons, using testosterone treatment (180 gel and 111 testosterone undecanoate (TU) injection users). Testosterone, estradiol and T/E ratio were assessed. In 10 volunteers we compared T/E ratios between gel-exposed and unexposed arms where the gel was applied on the venipuncture site or distal of the venipuncture site.

Using an upper limit of 0.60 for the T/E ratio, we found 12 samples (7.3 %, 95%CI 3.8-12.3 %) with an elevated T/E ratio in the gel group and one (0.9 %, 95%CI 0.0-4.9 %) in the TU group. In the explorative study application of testosterone gel on the venipuncture site was associated with a higher mean (range) testosterone concentration (33. 2 (17.0-50.0) vs 14.2 (7.2-21.0) nmol/L and a higher mean (range) T/E ratio (0.55(0.29-0.66) vs 0.23(0.18-0.29)) compared to samples taken from unexposed arms. This difference between arms could not be reproduced when applying gel distal to the venipuncture site.

Testosterone contamination of blood samples is rather common among testosterone gel users, and can be recognized by an increased T/E ratio. To prevent testosterone contamination, application of testosterone gel especially at the venipuncture site should be avoided. Clinicians and laboratory specialists should stay attentive to this common form of contamination and instruct patients accordingly.