Introduction: There is evidence that an early morning plasma cortisol (EMC) below <160 nmol/l is predictive of failing the SST and the corollary is seen with an EMC above >340 nmol/l. Using an EMC to screen patients for AI has been advocated, although there is a paucity of paediatric studies. Modern sensitive and specific cortisol assays make deriving local diagnostic thresholds important. We analysed our SST data since the introduction of a new cortisol assay to derive screening thresholds for SST and examined the relationship between the basal, incremental and peak plasma cortisol.
Methods: All SST performed between September 2014 and 2017 were retrospectively analysed. Cortisol quantification was performed on the Abbott Architect i1000 chemiluminescent immunoassay (CVs <5%). A pass for the SST is currently 430 nmol/l. Basal cortisol was used as a surrogate for EMC and correlation coefficients with increment and peak examined. Subgroup analysis was performed using sex and an age-approximate for pubertal status (09 and 1016 years of age). Positive and negative predictive values using a basal plasma cortisol of <160 nmol/l and >340 nmol/l respectively were calculated. Predictive values were calculated using different cuts offs.
Results: Overall 393 SSTs were included (209M, 184F, 175 prepubertal, 218 post-pubertal). The correlation coefficient for basal and peak cortisol was 0.63, (0.63 female, 0.62 male; 0.65 09 years, 0.66 1016 years). There was no relationship in any of the groups between basal and incremental cortisol (overall data correlation coefficient −0.061). Of the cohort 28% had basal cortisols <160 nmol/l of whom 58% failed the SST, PPV=0.58. Correspondingly 13% had basal cortisols >339 nmol/l, none of whom failed the SST, NPV=1. Moving the basal cut off to >320 nmol/l would have resulted in missing three patients who subsequently failed their SST.
Conclusions: There is a reasonably strong relationship between basal and peak cortisol on the SST. No relationship exists between basal and incremental cortisol. Subgroup analysis did not strengthen the correlations. On the Abbott Architect plasma cortisol assay an EMC of >339 nmol/l appears to safely predict passing the SST and <160 nmol/l yields a high PPV for failing the SST.
07 - 09 Nov 2018
British Society for Paediatric Endocrinology and Diabetes