What is the lab value for bioavailable testosterone?

Bioavailable Testosterone Lab Values

The lower normal limit for bioavailable testosterone in adult men is approximately 2.30-2.72 nmol/L (66-78 ng/dL) when measured by ammonium sulfate precipitation, based on a large reference population of healthy men aged 20-74 years. 1

Reference Ranges by Measurement Method

Measured Bioavailable Testosterone (Direct Assay)

• Lower normal limit: 2.30-2.72 nmol/L in men ≤39 years old 1
• This represents the assayed bioavailable testosterone (aBT) after ammonium sulfate precipitation, which is considered the reference method 1
• Values decline with age, with significant decreases observed in older populations 1

Calculated Bioavailable Testosterone

• Calculated values vary substantially depending on the algorithm used 2
• Mean calculated bioavailable testosterone ranges from 3.87 to 10.4 nmol/L depending on which published algorithm is applied 2
• The Sodergard algorithm produces the highest values (mean 10.4 nmol/L), while the Emadi-Konjin algorithm produces the lowest (mean 3.87 nmol/L) 2
• Calculated bioavailable testosterone at a threshold of 0.78 nmol/L showed diagnostic utility in PCOS studies (sensitivity 0.76, specificity 0.83), though this population differs from adult males 3

Clinical Context for Interpretation

When to Measure Bioavailable Testosterone

• Bioavailable testosterone measurement is most useful when total testosterone falls between 6.5-13.0 nmol/L (187-375 ng/dL) 4
• Outside this range, total testosterone alone provides sufficient diagnostic information 4
• In men with diabetes who have total testosterone levels close to the lower limit, free or bioavailable testosterone should be measured due to expected decreases in sex hormone-binding globulin 3

Key Measurement Considerations

• Morning fasting samples are essential for accurate assessment, as testosterone levels fluctuate throughout the day 5
• Bioavailable testosterone represents unbound testosterone plus albumin-bound testosterone, reflecting the readily dissociable and biologically available fraction 6
• Only 0.5-3% of circulating testosterone is free, with the remainder bound to SHBG (60%) and albumin (38%) 6

Algorithm Discrepancies and Pitfalls

Calculation Method Variability

• Different published algorithms produce results that differ by up to 2.7-fold 2
• The association constants used (Ks for SHBG and Ka for albumin) dramatically affect calculated values 1
• Standard constants (Ks = 1×10⁹ L/mol, Ka = 3.6×10⁴ L/mol) produce calculated values approximately 2-fold higher than measured values 1
• Optimized constants (Ks = 1.9×10⁹ L/mol, Ka = 2.45×10⁴ L/mol) provide better correlation with measured values 1

SHBG Confounding

• Many calculated bioavailable testosterone algorithms remain significantly associated with SHBG concentrations, introducing potential confounding 2
• Androstenediol (5-diol) acts as a competitive SHBG inhibitor and should be considered in calculations for improved accuracy 1
• SHBG alterations can significantly affect total testosterone concentrations, making bioavailable testosterone measurement critical in certain clinical scenarios 6

Diagnostic Thresholds in Clinical Practice

Hypogonadism Diagnosis

• Total testosterone <8 nmol/L (230 ng/dL) highly supports hypogonadism diagnosis 7
• Total testosterone >12 nmol/L (346 ng/dL) is likely normal 7
• The "gray zone" of 8-12 nmol/L (230-346 ng/dL) requires evaluation of free or bioavailable testosterone 7
• This gray zone represents approximately 90% of cases where bioavailable testosterone measurement adds diagnostic value 4

Cost-Effectiveness Strategy

• Restricting SHBG and bioavailable testosterone measurement to patients with total testosterone between 6.5-13.0 nmol/L enables reagent cost savings exceeding 55% 4
• Below 6.5 nmol/L or above 13.0 nmol/L, total testosterone provides sufficient information for ruling out hypogonadism 4