Assay-specific differences in lipemic interference in native and intralipid-supplemented samples.

Abstract

Lipemia is a potential cause of analytical interference (1)(2). Determinations of the lipemic index (L-index) or triglyceride concentrations are used to quantify lipemia (3)(4). The soy-based lipid emulsion, Intralipid, has been used to simulate lipemia in interference studies (2)(5), but without evidence of how well it simulates naturally occurring lipemia. Many serum proteins can be quantified by immunoturbidimetric assays (6). Lipemia interferes by altering light scattering (1). Manufacturers often provide guidelines for the maximum acceptable lipemia that have been established by interference experiments using Intralipid-supplemented samples. For the Modular Analytics P analyzer (Roche Diagnostics), the maximum allowable triglyceride values range from 4000 mg/L for the prealbumin assay to 20 000 mg/L for the haptoglobin assay. Sample turbidity is only weakly correlated with triglyceride concentration in patient samples (4). Thus, to test the validity of the lipemic thresholds, we directly compared interference from lipemic patient samples with interference induced by supplementation with Intralipid. We prepared pooled samples by mixing excess serum from two to five individual patient samples. Samples were assayed immediately or were stored at 4 °C for up to 2 weeks before testing. Samples were mixed by multiple manual inversions before analysis. The 16 pooled samples were simultaneously assayed for L-index, triglycerides, α1-antitrypsin, ceruloplasmin, haptoglobin, prealbumin, and transferrin on a Modular Analytics P 800 analyzer. All results are the means of duplicate measurements. The protein assays are non-particle-enhanced immunoturbidimetric assays. The primary assay wavelength is 700 nm and the secondary wavelength 340 nm, except for transferrin (secondary wavelength, 505 nm). All absorbances measured at these wavelengths were within the linear range of the photometer. Triglycerides were measured by a colorimetric enzymatic assay with a primary wavelength of 700 nm and a secondary wavelength of 505 nm. The L-index was determined by …