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

PurposeThe aim of this study was to measure subcutaneous tissue cortisol obtained by microdialysis (MD) in 35 mechanically ventilated septic patients.Materials and MethodsUpon 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.ResultsTotal 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.ConclusionsMicrodialysis 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.