Determination of reference intervals for 13 plasma proteins based on IFCC international reference preparation (CRM470) and NCCLS proposed guideline (C28-P,1992): trial to select reference individuals by results of screening tests and application of maximal likelihood method.

By using a new international reference preparation for the plasma proteins (CRM470) prepared by IFCC, we launched a project to determine reference intervals for 13 plasma proteins (immunoglobulins, compliment components, transferrin, alpha 1antitrypsin, alpha 1acid glycoprotein, haptoglobin, alpha 2macroglobulin, ceruloplasmin, albumin, transthyretin) in accordance with the guideline proposed by NCCLS. As reference individuals, 999 subjects were first selected through medical examination and a health-check questionnaire. However, there were many who had "abnormal" values in some of the 25 screening tests results measured simultaneously (serum enzymes, lipids, blood counts, etc.). Therefore, we adopted a criterion that those individuals who had "abnormal" values (beyond 99% confidence intervals) in the 25 tests would be excluded. The exclusion resulted in appreciable narrowing of the distributions in all 13 plasma proteins. This implied that "apparent" normality of major screening tests results may be used as a criterion to select truly relevant reference individuals. A parametric approach using a modified Box-Cox power transformation formula and the maximal likelihood estimation led to almost perfect normalization of the reference distributions on all the items tested. Since the nonparametric method proposed by the NCCLS guideline gave less reproducible results in our simulation study, the parametric method appears to be a method of choice for the calculation.