Criteria for monitoring carcinoembryonic antigen: variability of sequential assays at elevated levels.

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Abstract
Patients (55) with advanced cancer and elevated plasma carcinoembryonic antigen (CEA) levels > 20 ng/ml (Roche assay) were monitored by clinical parameters for disease activity and a daily plasma specimen was obtained and stored frozen. In 45 patients with evaluable metastatic lesions 35 were stable; 5 had progressive disease; and 5 had regressive disease. Plasma CEA in patients with stable disease showed an overall coefficient of variation [CV] of 13%. The CV did not differ according to various quantitative CEA levels from < 100 ng/ml to > 1000 ng/ml. The CV in responding and progressive disease patients ranged at 13%-63%. An analysis of CEA variability relative to the baseline CEA level was possible using the formula .sqroot.2 times the variability about the mean; this yields a value of .+-. 36% representing the range within which .apprx. 95% of sequential CEA levels would lie in the absence of a clinical change in disease. In 225 CEA determinations in stable disease patients, 6% demonstrated an increase beyond this level (36%) and none demonstrated a decrease of > 36% from the baseline level. Guidelines are established for the boundaries of change in plasma CEA that may applied as a criterion (in conjunction with standard objective disease parameters) for determination of tumor response to therapy.