Lamellar body count in amniotic fluid: a comparative study of four different hematology analyzers.

present study shows once again that standardization and/or recalibration of immunoassays in general is difficult to achieve with a common calibrator alone but needs split-sample measurements with a reference measurement procedure. This stems from the well-known fact that immunoassays may suffer from differences in behavior related to differences between the matrices of calibrators and those of real samples (1). Standardization/recalibration makes sense only when sufficient correlation of the investigated immunoassay is achieved with the comparison measurement procedure (or sufficient specificity). As shown in Table 1, this prerequisite was fulfilled in the present study because all assays had an excellent second-order correlation with the ID-LC-MS/MS measurement procedure (0.980 Ͻ r Ͻ 0.992). The outcome of the recalibration of the test systems , which was done by use of the respective second-order regression equations (Table 1), is shown in Fig. 1B. As can be seen from Fig. 1B, all test systems were successfully recalibrated by this approach. Standardization cannot solve specificity/interference problems in immunoassays. Indeed, in this study, three samples showed considerable sample-related effects in the immunoassays because of properties of the matrix. Investigation of the reasons to which the phenomenon can be attributed was beyond the scope of this study. The present ID-LC-MS/MS measurement procedure does not yet have the status of a reference measurement procedure. It is not part of a complete reference system comprising an international primary calibrator such as the IRP 84/510. As described, the ID-LC-MS/MS measurement procedure used here was calibrated with a commercial C-peptide preparation. This was done, on the one hand, for economic reasons; on the other hand, it was also done because we did not consider the current C-peptide IRP superior to the commercial calibrator. The IRP, being a recombinant product, contains an impurity of 10%, and the ampoule content was only nominally assigned the value of 10 ␮g by comparison with several commercial C-peptide preparations. Ideally, a primary calibrator should be certified in terms of mass units by techniques such as amino acid analysis and MS. Another reason that the ID-LC-MS/MS measurement procedure used here cannot be claimed as a reference measurement procedure is that, as part of the validation process, it should be assessed in a round-robin trial for its ability to satisfy predefined performance specifications in terms of trueness/accuracy and precision. Such an assessment should preferably be endorsed by an authoritative organization (8, 9). In conclusion, the present study demonstrates that …