Mail-in paper strip vs. microcolumn technique for measurement of glycosylated hemoglobin

We compared glycosylated hemoglobin (GHb) determined from capillary blood samples on paper strips with a standard microcolumn technique in a cross-sectional observational study with laboratories blinded to duplicate samples. Both the standard and the filter strip laboratories were provided with 80 uniquely identified blood samples from 40 individuals. Each laboratory ran duplicate analyses on each sample, yielding 160 GHb values. The within-laboratory correlations between blinded duplicates were 0.98 for the standard (microcolumn technique) and 0.94 for the filter paper (affinity technique) laboratories. The between-laboratory correlations ranged from 0.69 to 0.77. When classifying patients by quartile of glycemic control, the laboratories agreed on 60% of the patients. In an effort to identify sources of between-laboratory variability, varying quantities of blood were applied to strips and reanalyzed. Five microliter drops always yielded inflated estimates of GHb. These data suggest that the estimates of GHb obtained from mail-in paper strips, although internally consistent, differ in important ways from standard laboratory values, reemphasizing the need for caution in the interpretation of interlaboratory and intermethod comparisons.