A comparison of participant mean values of duplicate specimens in the CAP Chemistry survey program

Six paired specimens of lyophilized human serum were circulated to participants in the CAP Comprehensive Chemistry Program. The intervals between circulations ranged from 13 to 33 weeks. Each specimen was analyzed for 21 constituents, and the six pairs of specimens provided 126 paired sets of data for statistical comparison. The mean values for all constituents reported by the participants for each pair showed close agreement. The median difference between the two mean values for 126 pairs was 0.6% of the concentrations of the constituents. Twenty-six individual pairs showed differences that were sufficiently large to be significant (p = less than .0005). The majority of these stemmed from differences reported by participants using the SMA 12/60. Although significant, these differences were so small that they exerted no demonstrable effect on the dispersion of the results reported by the SMA 12/60 users. One vial pair showed a loss of glucose. All six pairs showed a loss of triglyceride. There was a suggestion of loss of cholesterol and uric acid in one pair. These differences were small. This study demonstrates that a comparison of mean values in a large Survey program can provide a very sensitive method of monitoring the stability of the specimens. The study also suggests the repeatability of mean values in a large Interlaboratory Survey program far exceeds clinical needs, and the agreement may be closer than can be obtained by any other system of measurement.