Objective
Cortisol cut‐offs can predict requirement for Synacthen stimulation tests (SST). We assessed the performance of a standard cortisol cut‐off (375 nmol/L) across the morning and compared this with a time‐adjusted cut‐off.
Design
Retrospective audit
Patients
Community reference set (n=12 550) and SST patients (n=757).
Measurements
In the reference population, time‐specific cortisol medians were calculated and used to convert cortisol to time‐adjusted Multiples of the Median (MoM). In 757 SST patients, the predictive performance of a standard cortisol cut‐off (375 nmol/L) and its time‐adjusted MoM equivalent were compared.
Results
Median cortisol decreased by ~30 nmol/L per hour between 0700 and 1200h. In the reference population, proportions below the 375 nmol/L cut‐off increased throughout the morning (range 35%‐64%), whereas using the time‐adjusted MoM cut‐off proportions were consistent (range 46%‐50%), with a 17% maximal difference in referral rates between the two cut‐offs after 1100h. A similar pattern was noted in the SST cohort. When a cortisol MoM cut‐off was used to predict SST success, the excess proportion of patients tested and misclassification rates were lower and more consistent than when the standard cut‐off was used. A median cortisol of 375 nmol/L equated to 444 and 313 nmol/L before 0800 and after 1100 h, respectively.
Conclusion
The use of a standard cortisol cut‐off results in 17% more patients being referred for SST later in the morning. A time‐adjusted cortisol cut‐off provides consistent and lower referral rates, whilst maintaining similar or better performance than a standard single cut‐off in predicting outcome of SST.