Introduction: As an alternative to cortisol assays in the serum or saliva to diagnose hypercorticisme we evaluated the ability of a commercial cortisol assay to assess capillary blood cortisol concentrations using routine procedures.
Methods: Capillary blood was obtained using a device used by diabetics to routinely assess capillary glucose levels and deposited on Schleicher-Scheull 903 blotter paper (routinely used for neonatal diagnosis of hypothyroidism, CAD…). In separate experiments, blood was sampled from veins into EDTA-containing tubes to allow precise volume distribution on the blotter paper (15 μL). Discs were punched from blotter paper (6.5 mm Ø). Two solid phase cortisol RIA (Gamma-Coat Diasorin) were performed: method A routine assay in serum, or method B modified procedure to assay in saliva.
Results: There was a significant relationship between serum cortisol and capillary blood cortisol whatever the method used: r2 0.83 & 0.87 (A & B methods respectively). A multivariate analysis did not show any influence of hematocrite.
Intra- and inter-assay coefficients of variation were: 13 & 5 and 13 & 11%, (A & B methods respectively).
Detection threshold (intra-assay CV <20%) were 40 & 2.7 nmol/l corresponding to 40 & 80 nmol/l serum cortisol levels (A & B methods respectively).
Conclusion: Capillary blood cortisol assessment is reliable enough to initially investigate diurnal rhythm or minute dexamethasone test in patients in which venous sampling in notoriously difficult. Advantages of method A are the low detection threshold and the procedure identical to serum assay allowing simultaneous management of serum and capillary samples; advantages of method B is to allow duplicate assay.