Understanding Free Testosterone by Dialysis
"Free testosterone by dialysis" refers to a laboratory method called equilibrium dialysis, which is the gold standard technique for measuring the unbound (free) fraction of testosterone in blood—this is not a treatment or therapy, but rather a diagnostic test.
What This Test Measures
Free testosterone by equilibrium dialysis is a specific laboratory assay that quantifies the biologically active, unbound testosterone in serum 1, 2. This measurement is particularly important because:
- Total testosterone alone can be misleading in patients with altered sex hormone-binding globulin (SHBG) levels, which commonly occurs in renal disease 1
- Only free (unbound) testosterone is biologically active and available to tissues 2
- Equilibrium dialysis is considered the reference standard method for measuring free testosterone, superior to calculated estimates or direct immunoassays 1, 2
The Laboratory Technique
The equilibrium dialysis method works by:
- Placing serum in a dialysis chamber separated by a semipermeable membrane 2
- Allowing unbound testosterone to pass through the membrane while protein-bound testosterone remains behind 2
- Measuring the testosterone concentration in the dialysate after equilibration, which represents the free fraction 2
- The process typically takes less than 1 hour and requires 0.6 mL of serum 2
Clinical Relevance in Dialysis Patients
Measuring free testosterone by equilibrium dialysis is particularly important in dialysis patients because this population has high rates of testosterone deficiency and altered protein binding:
Prevalence of Testosterone Deficiency
- Over 50% of male dialysis patients have low or low-normal testosterone levels, even before reaching end-stage renal disease 3
- Hemodialysis patients have significantly lower testosterone levels compared to peritoneal dialysis patients (39.5% vs 5.6% with deficiency) 4
- The dialysis technique itself may independently affect testosterone levels, with hemodialysis associated with greater testosterone elimination 4
Why Free Testosterone Measurement Matters in Renal Disease
- Protein binding is altered in chronic kidney disease, making total testosterone measurements unreliable 1
- Free testosterone correlates better with clinical outcomes including mortality risk in dialysis patients 5
- Free testosterone levels below certain thresholds predict higher mortality risk in hemodialysis patients (AUC = 0.788 for predicting death) 5
When to Order This Test
Free testosterone by equilibrium dialysis should be measured in dialysis patients when:
- Total testosterone levels are borderline or near the lower limit of normal (10-14 nmol/L or approximately 300-400 ng/dL) 1, 3
- The patient has obesity, which alters SHBG levels 1
- Clinical symptoms of hypogonadism are present (decreased energy, libido, muscle mass) despite normal total testosterone 1
- Confirming diagnosis before initiating testosterone replacement therapy 1, 6
Common Pitfalls to Avoid
- Do not rely solely on total testosterone in dialysis patients, as altered protein binding makes this measurement unreliable 1, 3
- Do not use calculated free testosterone estimates when equilibrium dialysis is available, as it is the gold standard method 1, 2
- Do not assume testosterone levels are normal in dialysis patients without testing, as over half have deficiency 3
- Do not overlook the impact of dialysis modality on testosterone levels, as hemodialysis is associated with greater testosterone deficiency than peritoneal dialysis 4