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The two largest laboratory testing agencies in the Unites States are LabCorp and Quest. If you are undergoing testosterone replacement therapy you will most likely be having your blood drawn and analyzed by either of these two laboratory agencies. Your clinician should be aware that there exist differences in the way each laboratory analyzes testosterone and how the normal references ranges of testosterone are reported.
LabCorp
In 2017 LabCorp changed their normal reference range of serum testosterone values from 348-1197ng/dL to the a much lower 264-916ng/dL. The reason for the change was to encompass more men who are obese in the population and are known to have a lower serum testosterone level. This change is direct reflection that more men in society are overweight leading to a new normal lower serum testosterone reference range.
LabCorp offers high pressure liquid chromatography and tandem mass spectrometry (LC/MS) to measure serum testosterone. This is the gold standard as maintained and reported by the CDC Hormone Standardizations.
LabCorp recommends total testosterone be measured between 800am and 12pm.
Quest
Similar to LabCorp, Quest laboratories also provide LC/MS as the gold standard measurement assay for serum testosterone. Yet despite both companies utilizing LC/MS the normal reference ranges of testosterone are different. According to the Quest website the normal testosterone reference range for men is 250-1110ng/dL. This is significantly different from LabCorp.
The importance of this difference, relies upon the diagnosis of Low-T that some insurance companies may rely upon. For example, if any individual has a testosterone value measured to be 255ng/dL, according to the Quest laboratory reference range this individual has a normal testosterone value. When this level is applied to the LabCorp reference range, this individual would be outside the reference range and be considered to have hypogonadism.
While the American Urologic Association (AUA) defines Low-T as less than 300ng/dL, obtaining coverage for medication from insurance companies can also rely upon the assay results as depicted above.
For this reason, a clinician should seek to treat the patient, sign and symptoms of Low-T, rather than numerical value of testosterone if within the low normal reference range.