DIAGNOSTICS


continuous monitoring. The automation of this process can improve laboratory throughput while increasing the reliability of the results produced. To correctly analyse HIL interference, absorbance readings at different strategically selected wavelengths supplement the calculation of the interference indices. C56-A recommends laboratories consider several parameters when selecting an HIL interference analysis method:3 n Interferant test concentration. The laboratory should consider the concentrations at which it tests for HIL interferants. The concentrations should be clinically relevant and cover the entire range of possible interference. The index value should increase as interferant concentration increases.


Normal Icterus


Icteric interference has classically been identified by the detection of a yellow pigmentation of the sample, caused by high levels of bilirubin. Most prevalent in neonatal departments, icterus can occur as a result of hepatic necrosis, sepsis or other pathological conditions.4 The greatest icteric affect can be seen at 460nm, causing interference in assays which measure between 400- 550nm including those for phosphate and creatinine. As bilirubin has high antioxidant activity, it freely reacts with hydrogen peroxide, causing a negative bias in assays for cholesterol, triglycerides and uric acid.1


Lipaemia


Lipaemia is defined as an increased turbidity of a sample due to high lipoprotein concentration. Commonly caused by insufficient fasting prior to sample collection, lipaemic interference affects as many as 2.5% of all patient samples.5


The increased turbidity


generates a light-scattering effect which interferes with assays proportionally to the size and quantity of the lipoproteins present. This causes a reduction in the effective spectral linearity of the assay, interfering with assays measured between 300-700nm, however, interference


Haemolytic Illustrations of normal, haemolytic, icteric and lipaemic samples.


increases as wavelength decreases.3 Diagnostic assays which measure NAD(P) H concentration at low wavelengths are particularly susceptible to lipaemic interference. Lipaemia can also act through other mechanisms such as increase erythrocyte debris, platelet leukocytes, fibrin clots, contaminated particulate matter, or through the absorption of hydrophobic analytes, reagents, or reaction products by lipid molecules.3 Other origins of lipaemic interference are high alcohol consumption, some medications, and hereditary conditions.


Detection of HIL interference C56-A Haemolysis, Icterus and Lipaemia/Turbidity Indices as Indicators of Interference in Clinical Laboratory Analysis; Approved Guidelines3


states:


“HIL indices should be measured on all samples which analytes are sensitive to haemolysis, icterus and lipaemia/ turbidity.” This includes a wide variety of assays from different disciplines and therefore, encompasses a large proportion of a laboratory’s testing capabilities, emphasising the importance of HIL interference. As visual detection of HIL interference has become obsolete, the onboard capabilities of many analysers to index these forms of interference require validation and


As visual detection of HIL interference has become obsolete, the onboard capabilities of many analysers to index these forms of interference require validation and continuous monitoring


50 Icteric Lipaemic


n Sample volume. Neonatal, geriatric, and critical-care patient samples are often supplied in very low volumes. Laboratories should consider the minimum sample volume required to determine an HIL index.


n Wavelengths and methods. Due to the large overlap in spectra of interferants, laboratories should consider the utility of wavelengths and methods selected.


n Number of indices. The number of indices provided by the HIL detection method should be considered. There is no recommendation for how many indices this should be, however laboratories should consider indices and related concentrations when choosing an HIL detection method.


n Read time. Laboratories should consider the test turnaround time for HIL detection and ensure it is practically applicable to its day-to-day activities.


Before results of any HIL detection method are used for patient samples, the specificity and sensitivity should be assessed at a minimum of two clinical- decision concentrations. This evaluation should include the sensitivity of the icterus index to haemoglobin and lipids, the haemolysis index to bilirubin and lipids and the lipaemic index to haemoglobin and bilirubin.3 In the presence of HIL interference, laboratories are responsible for the handling of the associated results and samples. Under no circumstances should an HIL index be used to correct patient results. Generally, if a sample is deemed to be subject to one or more of these types of interference, the laboratory should reject the result and dispose of the sample correctly. However, in some cases, cut-off values can be defined. For example, haemolysis has a less significant effect on samples with high analyte


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