Interlaboratory variation in a diaminopimelic acid assay: influence on estimated duodenal bacterial nitrogen flow

Samples of ruminal bacteria and duodenal digesta were collected from two dairy cows fed a 65% forage diet. Samples were sent blind to four laboratories for diaminopimelic acid analysis. Analyzed values differed among laboratories within sample type, and concentrations ranked as follows: laboratory D greater than laboratory A greater than laboratory B greater than laboratory C. Consideration of differences in actual procedures used among laboratories resulted in several hypotheses to explain some of the interlaboratory variation. Using diaminopimelic acid values from each laboratory to estimate duodenal bacterial nitrogen flow showed that laboratory D estimated a 17% higher flow than the average for laboratories A, B, and C, which were similar.