Evaluation of the interference due to haemoglobin, bilirubin and lipids on Immulite 2500 assays: a practical approach.

BACKGROUND: Interfering substances such as haemoglobin, bilirubin and lipids in a sample may lead to wrong interpretation of immunoassay results by the clinician. In general, there has been minor attention to these interferences on immunoassays, whereas these effects on chemical assays are frequently described. Information about interferences by haemoglobin, bilirubin and lipids on the Siemens Immulite 2500 assays in the instructions for use is falling short. METHODS: Interferents in patient samples can be measured reliably in a semi-quantitative way on most chemistry analysers and can be expressed in haemolysis-, icterus- and lipaemia-indices. As the Immulite 2500 cannot perform such measurements, samples are normally analysed without testing for the presence of interferents. Therefore, a study was carried out to examine these interferences on 24 Immulite 2500 assays. Samples were spiked with increasing concentrations of either haemoglobin, bilirubin or lipids. The haemolysis-, icterus- and lipaemia-indices were measured on a Synchron DxC 800 analyser. RESULTS: Based on analytical imprecision and intraindividual biological variation of each analyte, cut-off indices above which clinically significant interference exists were determined. We found clinically significant interference due to haemoglobin on ferritin and folate, by bilirubin on oestradiol and testosterone and by lipids on testosterone. CONCLUSIONS: Introducing cut-off indices prevents reporting of wrong Immulite 2500 results due to interference. Our results are applicable in laboratories using any chemistry analyser capable of reporting semi-quantitative concentrations of interferents.

A mouse genetic model for familial cholestasis caused by ATP8B1 mutations reveals perturbed bile salt homeostasis but no impairment in bile secretion.

Mutations in ATP8B1, a broadly expressed P-type ATPase, result, through unknown mechanisms, in disorders of bile secretion. These disorders vary in severity from mild and episodic to progressive with liver failure. We generated Atp8b1G308V/G308V mutant mice, which carry a mutation orthologous to that present in homozygous form in patients from the Amish index kindred for severe ATP8B1 disease. In contrast to human patients, Atp8b1(G308V/G308V) mice had unimpaired bile secretion and no liver damage, but showed mild abnormalities including depressed weight at weaning and elevated serum bile salt levels. We challenged the hepatobiliary metabolism of Atp8b1G308V/G308V mice by administering exogenous bile salts. Upon bile salt feeding, Atp8b1G308V/G308V mice, but not wild-types, demonstrated serum bile salt accumulation, hepatic injury and expansion of the systemic bile salt pool. Unexpectedly, this failure of bile salt homeostasis occurred in the absence of any defect in hepatic bile secretion. Upon infusion of a hydrophobic bile salt, wild-type mice developed cholestasis while Atp8b1G308V/G308V mice maintained high biliary output and more extensively rehydroxylated the infused bile salt. Increased bile salt hydroxylation, which reduces bile salt toxicity, may explain the milder phenotype in Atp8b1G308V/G308V mice compared with humans with the equivalent mutation. These results demonstrate the key role of Atp8b1 in bile salt homeostasis and highlight the importance of bile salt hydroxylation in the prevention of cholestasis. The mouse phenotype reveals that loss of Atp8b1 disrupts bile salt homeostasis without impairment of canalicular bile secretion; in humans this process is likely to be obscured by early onset of severe liver disease.