Is the freshness limit of the samples truly critical?
Yes. One of the primary reasons the CRMLN was established is that freezing, lyophilization, and other processes result in matrix effects that lead to such samples behaving differently from fresh patient samples. The CRMLN protocol, which calls for analysis of fresh samples by the manufacturer’s test method, is designed to allow manufacturers to verify their calibration so they obtain the correct result for patient samples. For total cholesterol, the levels of cholesterol in freshly collected samples probably do not change for several days. However, some small studies and anecdotal information indicate that HDL cholesterol and LDL cholesterol change upon storage, so the freshness guidelines are critical (for both the test methods and for the reference methods). The CRMLN has devised a “Sample Stability Protocol” to assist manufacturers in evaluating the stability of frozen samples over time. A copy of the protocol is available on this Web site.
