DEFINING MORNING SERUM CORTISOL CUT-OFF VALUE IN PREDICTING NORMAL RESPONSE TO SHORT SYNACTHEN TEST

A SINGLE CENTRE RETROSPECTIVE STUDY

Abstract

INTRODUCTION
Ambulatory morning serum cortisol<100 nmol/L has been demonstrated in some studies to be predictive of adrenal insufficiency with reasonable accuracy. However, data on the serum cortisol level that predicts adrenal sufficiency is inconclusive. This study aims to determine the serum morning cortisol value best predictive of normal response to Short Synacthen Test (SST) in our patient cohort.

METHODOLOGY
A retrospective study was conducted on SSTs performed from September 2016 to December 2018 in Endocrine Unit, Hospital Sultanah Aminah Johor Bahru (HSAJB). Relevant information including demographic data, indications for SST, baseline morning serum cortisol levels and SST results were derived from clinic notes as well as cobas® infinity central laboratory system. Normal response to SST was defined as a 30-min or 60-min post-Synacthen cortisol of >500 nmol/L.

RESULTS
Thirty [median age 55 years (IQR 32.50, 65.25), 53.3% male] out of 55 patients who underwent SSTs from September 2016 to December 2018 were included in the analysis. Exogenous steroid usage constituted one-third of the indications for SST; followed by pituitary tumour +/- surgery (30%) and Rathke’s cleft cyst (6.7%). Fourteen patients (46.7%) demonstrated normal response to SST. Baseline morning cortisol was demonstrated to be an unsatisfactory tool to predict adrenal sufficiency in our patient cohort [area under curve (AUC) 0.518, 95% CI 0.306–0.730]. The cutoff level most predictive of adequate adrenal reserve was identified to be >266 nmol/L with a specificity of 93.7%, but at the expense of a very low sensitivity (14.3%). Age was a statistically significant predictor of adrenal reserve. Increasing age was associated with a reduction in the likelihood of exhibiting adequate SST response (odds ratio 0.860, 95% CI 0.741–0.997, p=0.046).

CONCLUSION
Baseline morning cortisol has a low overall predictive value of passing SST in our patient cohort. Rising age increases the probability of failing SST.