Carbon dioxide combining power/potassium ratio in etiological diagnosis of Cushing's syndrome

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Objective To evaluate carbon dioxide combining power/potassium ratio (CO2CP/K)in etiological diagnosis of Cushing's syndrome. Methods The clinical data 489 patients with pathohistologically confirmed Cushing's syndrome treated in PLA General Hospital between 1993 and 2015 were retrospectively reviewed. Among 489 patients, there were 246 cases of adrenocorticotropic hormone(ACTH) nondependent Cushing's syndrome and 243 cases of ACTH dependent Cushing's syndrome, including 214 cases of Cushing's disease (CD) and 29 cases of ectopic ACTH syndrome (EAS). Receiver operating characteristic(ROC) curve was used to evaluate efficacy of CO2CP/K for etiological diagnosis and the cut-off value and corresponding sensitivity and specificity were analyzed. Results Blood potassium levels were lower in ACTH nondependent group than those in ACTH dependent group[(3.46±0.77) vs.(3.69±0.57)mmol/L, t=3.433, P=0.001]. Bicarbonate, base excess and CO2CP/K (10.0±5.5 vs. 7.6±2.1) were higher in ACTH nondependent group than those in ACTH dependent group (t=-3.612, P=0.000). There was no significant difference in serum sodium, carbon dioxide binding force and pH value between two groups (P>0.05). Blood potassium [(2.47±0.63) vs.(3.62±0.67)mmol/L] and chlorine levels were lower in EAS group than those in CD group. Bicarbonate, base excess and CO2CP/K (15.6±8.0 vs. 7.8±2.4) were higher in EAS group than those in CD group (t=- 10.145, P=0.000). There was no significant difference in serum sodium, carbon dioxide binding force and PH value between two groups (P>0.05). Taking 8.95 as cut-off value the sensitivity and specificity of CO2CP/K in diagnosis of ACTH dependent Cushing's syndrome were 0.34 and 0.85, while taking 9.09 as cut-off value the sensitivity and specificity of CO2CP/K in diagnosis of EAS were 0.89 and 0.78, respectively. Conclusions The value of CO2CP/K is limited in distinguishing between ACTH dependent and non-ACTH independent Cushing, s syndrome. It may distinguish CD from EAS to some extent, indicating that it might be used as the screening method for etiological diagnosis. Key words: Cushing's syndrome; Hypokalemia; Etiology; Diagnosis; Adrenal gland neoplasms