Accuracy of the Vermeulen Equation with Elevated SHBG
The Vermeulen equation is reasonably accurate for calculating free testosterone even with your elevated SHBG of 90 nmol/L, though it will likely overestimate your true free testosterone by approximately 19%. 1
Understanding Your Specific Situation
Your free testosterone of 0.5 nmol/L falls within the reference range (0.117-0.62 nmol/L), but with SHBG at 90 nmol/L—which is substantially elevated—the calculated value requires careful interpretation. 2
Performance of the Vermeulen Equation
The Vermeulen equation consistently overestimates free testosterone by a median of 19% compared to the gold standard equilibrium dialysis method, but importantly, this overestimation remains relatively stable across different SHBG levels. 1
The equation demonstrates strong correlation with equilibrium dialysis (r = 0.986) and is largely independent of SHBG, albumin, and total testosterone levels, making it the most robust calculation method available. 1, 3
Among all calculation methods, the Vermeulen equation shows the highest rank correlation (0.89) with directly measured free testosterone, outperforming alternative equations like Ly (0.74) and Zakharov (0.62). 1
Clinical Implications for Your Case
Given the 19% overestimation, your true free testosterone measured by equilibrium dialysis would likely be approximately 0.42 nmol/L (still within normal range, but closer to mid-range rather than upper range). 1
The Endocrine Society recommends the Vermeulen equation as the standard calculation method for clinical practice, acknowledging it as an acceptable alternative when equilibrium dialysis is not available. 4, 2
If you have symptoms of hypogonadism despite your calculated free testosterone appearing normal, direct measurement by equilibrium dialysis would provide more accurate results and should be considered, particularly given your elevated SHBG. 2, 5
When to Question the Calculation
Morning serum samples should be used for testosterone assessment due to significant diurnal variation, and repeat testing is necessary to confirm the pattern since single measurements can be misleading. 2, 5
If hypogonadism is suspected despite normal calculated free testosterone, further evaluation including luteinizing hormone and follicle-stimulating hormone measurements may be warranted. 2
Clinical symptoms should be considered alongside laboratory values when evaluating testosterone status in men with abnormal SHBG levels like yours. 2