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Do enzymatic analyses of serum triglycerides really need blanking for free glycerol?

In their study of 419 inpatient and 339 outpatient triglyceride samples, Jessen et a!. (1) conclude that “the performance of routine glycerol blanks is clinically unnecessary under most circumstances,” pointing as they do to their finding of <15% false increases in 92% and 99% of inpatients and outpatients, respectively. The accompanying editorial (2) reiterates this advice for individuals with normotriglyceridemia but advocates routine glycerol blanking for triglyceride values >2.82 mmol/L (2500 mg/L). We recently addressed this question in a multicenter study of the effectsof gemfibrozil in type U (non-insulin-dependent) diabetics with hypertriglycendemia. To determine the accuracy of the standard (nonbianked) assay, the core laboratory for this study, SmithKline Beecham Clinical Laboratories,agreed to perform standard and glycerol-blanked triglyceride determinations on duplicate samples from a subpopulation of patients whose triglyceride values determined by the standard methodology ranged from 3.39 mmol/L to an upper study limit of 5.65 mmol/L (3000-5000 mg/L). Serum samplesdrawn across the 49 study sites were transported at ambient temperature by courier to the SmithKline Beechain Clinical Laboratories in Van Nuys, CA. Nonblanked triglyceride values were obtained by using routine Beckman reagents to produce a quinoneimine dye and comparing the absorbance value at 540 nm with that produced by a known calibrator. Glycerol-blanked values were obtained with a commercially available reagent set (TriglyceridesGB, cat. no.877557; Boebringer Mannheim Diagnostics, Indianapolis, IN) in which free glycerol is eliminated before the hydrolysis of triglycerides by the lipase reagent; otherwise, this methodinvolves the same biochemical pathway as the Beckman method: a glycerol kinase/glycerol-3-phosphate oxidase coupled enzyme method (3). Results (Figure 1) indicatethat measurements obtained with the standard method were significantly (P <0.01) higher than those obtained with the blanked method. However, the correlation between the two methods (r = 0.91) was also significant (P <0.01), indicating that the current routine methodwas a good predictor of values obtained with the blanked method. CURRENT METHOD Fig. 1. Comparison of standardandglycerolblanked assaysfortriglyceride, mmoL/L(n = 152)