Why do immunoassays for tumour markers give differing results?--a view from the UK National External Quality Assessment Schemes.

External Quality Assessment schemes can provide unique insight into how current methods are performing in the field. Results from the UK National External Quality Assessment Schemes, which monitor performance of immunoassays for a number of tumour markers and peptide hormones in serum, show that in spite of considerable improvements resulting from increased assay automation, major discrepancies in results obtained are still observed. Reasons for this include poor calibration, use of antibodies of differing specificity, and vulnerability to clinically relevant interferences. Variation in quoted reference ranges is also a cause of concern. Each of these important aspects of performance will require attention if improved between-method and between-laboratory agreement is to be achieved.