Minimally processed fresh frozen human reference sera: preparation, testing, and application to international external quality assurance.

The preparation of unmodified or minimally processed fresh frozen human sera is described, as well as the previous use of such sera, e.g. in Nordic and international external quality assurance (EQA) activities. The unmodified serum is prepared from fresh donors' blood collected in dry bags and allowed to coagulate. The serum is collected "on the clot", pooled, filtered, mixed, dispensed in polypropylene vials and frozen at -80 degrees C without further processing. Some batches were slightly modified by spiking or dilution. Critical steps of the production and use of the sera are described and improvements are discussed. A total of 34 different batches have been prepared since 1985. Results from homogeneity and stability studies are presented. The studies cover 18 routine components in serum stored at +4 degrees C to 37 degrees C for up to 34 days. Good stability was observed for storage of all components, with the exception of triglyceride. Amylase, creatininium, glucose, gamma-glutamyltransferase, urate (and perhaps carbamide) showed deterioration after 13 days of incubation at 37 degrees C. The long-term stability at -80 degrees C is reviewed and new data are presented, e.g. as consensus values from EQA schemes, where the same serum has been sent out three times over 5 years, and from reference measurement procedure values that have been assigned twice with an interval of 5 years. Furthermore, a 10-year stability study has been started.

The Nordic Trueness Project 2002: use of reference measurement procedure values in a general clinical chemistry survey.

Up to 136 laboratories participated in a joint effort to assess the trueness of routine measurements for 14 serum components. An unmodified, fresh-frozen human serum ("IMEP-17 Material 1"), produced for an international inter-laboratory comparison, served as the "master material". The serum had assigned values of the highest available metrological quality, and is assumed to involve no or negligible commutability problems. The material was used in the assignment of traceable values to two other reference sera, "CAL" and "X", through parallel measurements on the three materials according to a common protocol. In this transfer process, uncertainty estimates were provided for all values. The material CAL had been supplied with reference measurement procedure values in 1997, and the two sets of assigned values agreed well. A lyophilized control serum "HK02" was also included in the routine analysis series. It, too, had assigned values based on reference measurement procedures. Significant matrix effects were found. The project has provided: Assigned traceable values for 14 components in a fresh-frozen serum, available to Nordic laboratories for the coming years as "NFKK reference serum X"; Confirmation of earlier assigned reference measurement procedure values for a number of components in CAL, the main calibrator in the Nordic Reference Interval project (NORIP). The transferred values will now serve as the primary reference.; Evidence of long-term stability ( > or = 5 years) of the fresh-frozen serum CAL when stored at -80 degrees C; Evidence of substantial matrix effects in the processed serum HK02. The findings should be used to discuss to what extent reference measurement procedure values are useful and cost-efficient for this type of material.