Interference due to lipaemia in routine photometric analysis--survey of an underrated problem

Traditionally external quality assessment schemes (EQAS) distribute samples spectrally similar to a normal serum. This does not allow laboratories to assess the performance of their methods at measuring lipaemic samples. The Irish EQAS distributed a sample with a supplied lipid diluent. A sample of the same batch had previously been circulated as a normal distribution to obtain baseline values. Most analytes were affected by the lipaemia. The degree of interference was instrument/method dependent and a wide range of values was reported for each analyte: total protein 51-134 g/L (baseline 65 g/L); albumin 33-56 g/L (baseline 42 g/L); calcium 2.12-6.10 mmol/L (baseline 2.34 mmol/L); urate 38-1100 mumol/L (baseline 195 mumol/L); creatinine 6-199 mumol/L (baseline 140 mumol/L); glucose 4.2-9.5 mmol/L (baseline 4.8 mmol/L); total bilirubin 1-192 mumol/L (baseline 36 mumol/L); urea 1.1-8.1 mmol/L (baseline 3.8 mmol/L); aspartate aminotransferase (AST) 12-105 iu/L (baseline 44 iu/L); iron 4.3-107 mumol/L (baseline 26.8 mumol/L); inorganic phosphorus 1.12-4.96 mmol/L (baseline 1.59 mmol/L); and cholesterol 2.5-17.1 mmol/L (baseline 4.1 mmol/L). In situations where no clearing of the lipid occurs in the reagent, interference can be minimized by taking a sample blank reading or using a kinetic procedure. If clearing of the lipid does occur, time must be allowed for this to be completed before analytical readings are taken.