Fig. 2.

Differences between repeat and reference serum OD ratios at 7 days post-collection for dried blood spots (DBS) and serum measurements. A. The difference between the mean OD ratio from a DBS collection (three measurements) and the OD ratio of the reference serum from the same subject (y-axis values) as a function of the reference serum OD ratio (x-axis). The results shown are based on a linear mixed model analyzing the three repeated OD ratios for each sample from both the DBS and Serum Repeat (SR) studies. The means (dark lines) were estimated using a linear mixed model, and 95% confidence intervals are shown as the shaded area. The red line at the bottom indicates the region where the difference from the reference serum OD ratio differs significantly between the DBS and SR studies. B. Estimated mean differences as indicated in panel A, but at reference serum measurements near the analytical decision points (OD ratios of <0.8, 0.80–1.09 and ≥ 1.1 are interpreted as negative, equivocal, and positive, respectively). Estimated means and 95% confidence intervals are shown. The red line at the bottom indicates a statistically significant difference between serum and DBS. C. The estimated variance among replicate OD ratio values for all subjects, with the variance depending on the reference serum OD ratio. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)