VALUE OF 30 AND 60-MINUTE CORTISOL VALUE DURING SHORT SYNACTHEN TEST – CAN WE DO AWAY WITH ONE OR THE OTHER?

Abstract

INTRODUCTION

The short synacthen test (SST) using 250 mcg synthetic ACTH is the most widely used test to identify adrenal insufficiency (AI). The standard testing protocol that requires both 30 and 60-minute cortisol values increases resource utilisation and cost. We examine the utility of 30-minute versus 60-minute single time point cortisol values in identifying AI, compared with the convention of values at both time points.

METHODOLOGY

A retrospective analysis of SSTs done at a single centre between 2018-2021 was done. Serum cortisol was measured at 0, 30 and 60 minutes after 250 mcg of intravenous synacthen. Adequate response was defined as cortisol values of ≥500 nmol/L at either or both time points. We compared 30 and 60-minute values against overall response during SST.

RESULTS

A total of 360 patients (age: 61.5±17.7years, 44% male) were studied. Indications for SST were exogenous steroid use (41%), pituitary disease (13%), low morning cortisol (27%), hyponatremia (6%), hypotension (4%) and others (9%). Median (IQR) cortisol values at 0, 30 and 60 minutes were 250 (165-371), 581 (427-724), and 651 (479-819) nmol/L respectively. Adequate response was seen in 217 (60.3%) while 96 (26.7%) had inadequate response at both 30 and 60 minutes respectively. Inadequate response at 30 minutes but adequate response at 60 minutes was seen in 42 (11.6%), while 5 (1.4%) had adequate response at 30 minutes but inadequate response at 60 minutes. Using 60-minute cortisol alone was found to have a sensitivity of 98.1% and specificity of 100%, with 100% positive predictive value and 95.1% negative predictive value.

CONCLUSION

The probability of overdiagnosis of AI is significantly higher if only 30-minute cortisol values were to be considered without the 60-minute cortisol values. This study highlights the importance of measuring the 60-minute cortisol value to avoid misclassification of AI.