Analytical performance of free testosterone calculated by direct immunoluminometric method compared with Vermeulen equation: results from a clinical series.

Abstract

Abstract Introduction: Testosterone is a hormone crucial for primary and secondary sexual development in both males and females. Free testosterone (FT) represents the biologically active form of T, and its measurement holds significant importance in clinical practice. While equilibrium dialysis or ultrafiltration are considered the gold standard for FT assessment, these methods are expensive and not widely accessible. As an alternative, the Vermeulen formula is a commonly utilized calculated method. Methods: This clinical study involved 190 consecutive patients, comparing FT levels obtained through direct immunoluminometric assay and the Vermeulen formula. The comparison was performed using Passing-Bablok, Deming regressions, as well as the Bland-Altman plot. Sensitivity, specificity, accuracy, positive predictive value, and negative predictive value were assessed. Results: The calculated method, employing the Vermeulen formula, was considered the gold standard. Passing-Bablok regression indicated a good agreement between the two methods, with slopes close to 1. The Bland-Altman demonstrated overall agreement, but a potential proportional bias was observed in females. Deming regression confirmed excellent agreement and reliable estimates. Sensitivity and specificity analysis revealed that the direct method had a sensitivity of 75.0% and specificity of 93.4% considering all patients. However, sensitivity improved to 81.0% in males and dropped to 18.2% in females, likely due to the low number of true positive cases. Conclusion: In conclusion, the direct method exhibited comparable performance to the calculated method, but caution should be exercised when interpreting results, particularly in females. Further studies are necessary to validate its sensitivity and specificity in larger series.