Probabilistic application of plasma carcinoembryonic antigen assay in cancer patients.

Plasma carcinoembryonic antigen (C.E.A.) levels in inpatients proved at necropsy to be cancer free were used to assess the ability of the C.E.A. assay to distinguish benign and malignant disease. The patients had a mean C.E.A. level significantly greater than that for young healthy people. In view of the considerable overlap of the ranges of plasma C.E.A. concentration in cancer patients and patients with non-malignant disease a probabilistic interpretation is advocated rather than the use of a simple cut-off between positive and negative. On the basis of the cancer-free control group, 19 out of 64 untreated patients with various solid tumours had plasma C.E.A. levels considered to correspond to a greater than 95% probability of cancer.