[Differences in Measured Values among Homogenous Assay Reagents of LDL-C in LP-X Positive Serum Samples]

The LDL-C level measures with homogeneous (direct) assays in almost of clinical laboratories. Several reports however showed differences in measured values among the assay reagents. We investigated the differences in LDL-C values among direct assays and Friedewald formula (F-f) in 58 LP-X positive serum samples from jaundice patients by comparing LDL-C values measured by anion-exchange chromatography (AEX-HPLC), largely comparable to ultracentrifugation method. Changes in LDL-C values during the treatment of 8 patients were also investigated. Direct assay reagents from Sekisui Medical (S-r), Denka-Seiken (D-r), Wako Chemical (W-r), and Kyowa Medics (K-r) were used for comparison. F-f, S-r, and D-r correlated with AEX-HPLC with r values < 0.6 while W-r and K-r correlated with AEX-HPLC with r-vales > 0.6. Two samples in which F-f values provided 500 mg/dL plus bias to AEX-HPLC (LDL-C value of 220 mg/dL) demonstrated increased levels of IDL-C before treatment. LDL-C values (S-r and D-r) of the 2 samples were relatively high and near to F-f data while LDL-C values (W-r and K-r) were relatively low and close to AEX-HPLC data. The jaundice treatment decreased LDL-C values (S-r and D-r) and converged to 220 mg/dL, indicating that S-r and D-r might react markedly to IDL. These changes were consistent with decreases in serum free cholesterol and phospholipid in support of LP-X. By contrast, W-r and K-r data showed upward tendency and also converged to 220 mg/dL. These results suggest that LDL-C direct assay reagents would be classified into 2 groups with respect to the reagent reactivity to LP-X.