Effects of varying dilutions of quality control sera on precision of analytes

Precisions obtained during an "unknown" quality control program using lyophilized serum reconstituted with varying amounts of diluent were compared with precisions achieved with the manufacturer's recommended dilution for 23 commonly measured analytes, including enzymes and radioimmunoassays, for four laboratories over two years. Except for bilirubin and possibly calcium and alkaline phosphatase, no tendency to deterioration of precision was observed as a result of varying the volume of diluent. The range of dilutions was from 92% to 108% of the recommended volume.