Pre-test Cortisol Levels in Predicting Short Synacthen Test Outcome: A Retrospective Analysis

OBJECTIVE: Short Synacthen tests (SSTs) are expensive, dependent on Synacthen availability, and need supervision. To reduce SST testing. we examined the utility of pre-test cortisol (Cort0) and related parameters in predicting outcome. DESIGN AND MEASUREMENTS: We retrospectively examined the following in all SSTs; (i) Cort0 (ii) indications (iii) and time and place of testing. Receiver operated characteristic (ROC) curves were devised for Cort0 to obtain the best cut-off for outcome prediction in those who had SSTs between 8 and 10 am (Group 1) and at other times (Group 2). RESULTS: Of 506 SSTs, 13 were unsuitable for analysis. 111/493 SSTs (22.5%) were abnormal. (1) ROC curves predicted - (a) SST failure with 100% specificity when Cort0 was <= 124 nmol/L (Group 1), or <= 47 (Group 2); (b) a normal SST with 100% sensitivity when Cort0 >= 314 nmol/L (Group 1) and >= 323nmol/L (Group 2). (2) There was significant correlation between Cort0 and 30-minute cortisol (r(s) = 0.65-0.78. P < .001). (3) Median Cort0 was lower in those who failed SSTs compared to those who passed (147 vs 298nmol/L respectively. P < .001). (4) SST failure was commoner in Group 1 vs 2 (P= .001). (5) There was no difference in outcome between out-patient and inpatient SSTs. (6) SST failure was most common for 'steroid related' indications (39.6%, P < .001). CONCLUSIONS: This study indicates that (1) Cort0 >= 323 (Group1) and >= 314 nmol/L (Group 2) predicted a normal SST with 100% sensitivity; (2) Using these cut offs 141/493 (28.6%) tests may have been avoided; (3) supporting evidence should be considered in those with a lower pre-test predictability of failure.