Interstitial cortisol obtained by microdialysis in mechanically ventilated septic patients: correlations with total and free serum cortisol.

PURPOSE: The aim of this study was to measure subcutaneous tissue cortisol obtained by microdialysis (MD) in 35 mechanically ventilated septic patients. MATERIALS AND METHODS: Upon intensive care unit admission, an MD catheter was inserted into the subcutaneous tissue of the thigh. Cortisol (CORT) was determined in a 5:00 to 9:00 am microdialysate sample collected within 72 hours. Concurrently, serum total (T-CORT) and free CORT (F-CORT) were measured. The Acute Physiology and Chronic Health Evaluation (APACHE II) and Sequential Organ Failure Assessment scores were calculated. Both T-CORT less than 10 µg/dL and F-CORT less than 0.8 µg/dL were considered as indicating critical illness-related corticosteroid insufficiency. Adrenal adequacy was defined as T-CORT greater than 20 µg/dL or F-CORT greater than 2.0 µg/dL. RESULTS: Total CORT correlated significantly with F-CORT (rs = +0.8, P < .0001). Microdialysis CORT had a lower correlation with T-CORT (rs = +0.6, P < .0001) and F-CORT (rs = +0.7, P < .0001) and a weak correlation with APACHE II score (rs = +0.4, P < .01). On the basis of MD-CORT, the patients were divided in quartiles. Although the median F-CORT and T-CORT levels were significantly different (P < .001) among the MD-CORT quartiles, there was a considerable overlap between the subgroups. All patients with T-CORT less than 10 µg/dL and all but 3 patients with F-CORT less than 0.8 µg/dL had tissue CORT in the lower quartile. However, only 50% and 58% of patients with adequate T-CORT and F-CORT levels, respectively, had concordant MD-CORT in the highest quartile. CONCLUSIONS: Microdialysis CORT levels correlate moderately with circulating T-CORT and F-CORT. Of note, several patients presented with discrepant measurements between interstitial and circulating CORT concentrations. Thus, interstitial CORT measurements represent an additional tool to investigate the tissue CORT availability in critically ill patients.