A study of conditions for the elution of apo A1 and B lipoproteins from dried blood spots has led to the development of an apo B/A1 ratio assay with results for dried blood spots which are comparable with serum assays. This assay has been developed to be suitable for large scale population screening. The concept of measuring ratios for co-eluting blood constituents improves the accuracy and precision of blood spot assays and opens up the possibility that patients could take their own blood sample and send it to the laboratory by post.