Biological variation and analytical imprecision of CA 125 in patients withovarian cancer

Despite the availability of serial data on CA 125 in ovarian cancer, the pr oblem of interpreting a change over time is still unsolved. Changes in mark er concentrations are due not only to patients improving or deteriorating b ut also to analytical imprecision and normal intra-individual biological va riation. The aim of this study was to assess the analytical imprecision (CV A) and the intra- and inter-individual biological variation (CVI and CVG, r espectively) of CA 125 in a group of 26 patients with clinically stable ova rian cancer. Furthermore, the critical difference for a change between two consecutive CA 125 concentrations calculated as root2 x Z x (CVA2 + CVI2)(1 /2) (Z = 1.65 for unidirectional and 1.96 for bidirectional changes, p less than or equal to0.05) and the index of individuality calculated as ((CVA2 + CVI2)/CVG2)(1/2) were estimated. After the exclusion of outliers, CVA and the average CVI and CVG were 12.1%, 24.0%, and 43.1%, respectively. The in dex of individuality was 0.62 and the critical difference calculated for un idirectional changes was 62.6%. CVA and CVI contribute considerably to the variation in serial results and should, therefore, be included in the crite ria for serum tumor marker assessment during monitoring of patients with ov arian cancer. The cut-off value of CA 125 is of minor value in detecting un usual results for an individual subject, when previous measurements from an individual are available. These measurements should be preferred as refere nce for interpretation of new results.