Large lipid particles may be removed from samples by ultracentrifugation. However, this may result in the sedimentation of the large molecular protein masses of fibrinogen or factor VIII/von Willebrand complex. Another method that may clarify plasma is by adding n-hexane to plasma samples to clear the lipids. This would have to be verified by laboratories to determine if the analyte of interest is affected.7
The interference of lipemia may also be minimized by using higher dilutions. When using immunoturbidimetric methods for automated, quantitative D-dimer testing and von Willebrand factor activity and antigen, the higher the dilution that can be used, the less the chance of lipid-based interference. Samples diluted 1:20 may show lipid interference; this may be removed by increasing the dilutions up to 1:400.6
Icteric plasma contains high levels of bilirubin. Normal levels of bilirubin are about 0.5 mg/dL. In cases of hyperbilirubinemia, levels will exceed 1.5 mg/dL and plasma will become affected.
Icteric plasma samples have a high prevalence in samples from patients in the intensive care unit, as well as gastroenterology, surgical and pediatric patients. Concentrations of bilirubin greater than 2.5 mg/dL can lead to clinically relevant changes of anti-thrombin. Higher concentrations can interfere with other coagulation tests.8
Studies have demonstrated that using higher wavelengths at 570 nm on samples that are icteric demonstrate concordance to electromechanical methods, making this an alternative method to measure these samples.9
Optical vs. Mechanical Methods
Laboratories use either photo optical or mechanical methods to measure coagulation results. There have always been questions as to which methodology has an advantage over the other in samples with hemolysis and lipemia or icteric samples. Optical clot detection systems have the capability to use multiple wavelengths to try to eliminate issues associated with visual interferences.
A large study compared more than 2,000 samples using both methodologies. Of those samples, 26.5% had visual interferences. The study demonstrated that results obtained by the photo-optical detection system are as reliable and statistically equivalent as those obtained using the mechanical detection system. The correlation between the analyzers resulted in an r>0.96 for all samples and an r >0.98 on samples that were turbid.10
Another small study looked at lipemic samples (n=10) using a mechanical method of clot detection and a photo-optical method of clot detection following ultracentrifugation. There was no difference in APTT and PT results when comparing the ultracentrifuged samples read by the photo-optical method and the mechanical methodology.6
Donna D. Castellone is clinical project manager, Siemens Healthcare Diagnostics, Tarrytown, NY, and an ADVANCE editorial advisory board member.
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