In recent years, there has been increased scrutiny of preanalytical errors, which account for as much as 70 percent of all errors in laboratory medicine diagnostic testing. In clinical chemistry testing, a subset of these errors is caused by preanalytical interference, i.e., compounds or molecules that lead to erroneous results by interfering with assays or causing physiological changes to the serum or plasma composition. The most common of these interferences include hemolysis, lipemia, and icterus. While many clinical labs have developed strategies to detect these interferences, strategies for resolving interferences are less ubiquitous. In this article, we’ll discuss three common interferences and provide you with new strategies to resolve them.
What is hemolysis and how does it affect lab tests?
Hemolysis is defined by the release of hemoglobin and intracellular components from erythrocytes into serum or plasma. While this phenomenon can be characteristic of a pathology, such as hemolytic anemia, it is most often a result of in vitro disruption of red blood cells. Hemolysis can occur during sample collection, transportation, and/or storage. Detection can be assessed visually (usually qualitative) or photometrically (usually quantitative by measuring free hemoglobin in the specimen).
Preventing interference caused by hemolysis
Interference due to hemolysis is typically caused by the release of intracellular material, which falsely elevates serum/plasma concentrations of certain analytes, such as potassium and lactate dehydrogenase, while diluting others, like sodium. Because hemoglobin absorbs light at 340–400 nm and 540–580 nm wavelengths, it can also prevent the accurate photometric measurement of analytes in specific assays. As a result, there is nothing you can do to analyze a hemolyzed sample. Instead, focus on prevention and have systems and recommendations in place to ensure the next samples are not hemolyzed.
Communicating with phlebotomists and nursing personnel about correct blood draw technique is a crucial first step. Factors such as tourniquet time, fill volume, needle size, and specimen mixing can all contribute to hemolysis.
For example, blood draw by intravenous catheter (IV) results in significant hemolysis. Studies have indicated venipuncture to be generally superior, reducing hemolysis by as much as 84 percent compared to IV. Thus, ensure the clinical team uses appropriate needle sizes and correct draw speed, completely fills the tubes, and gently mixes the blood to avoid hemolysis.
Many hospitals use pneumatic tube systems (PTS) to rapidly transport specimens across hospitals to the processing laboratories. While convenient, these systems expose specimens to extremely damaging g-forces that can easily destroy erythrocytes. While walking all samples directly to laboratories is impractical, transportation protocols should call for this measure when dealing with particularly precious samples or in cases where hemolysis is a persistent problem. Additional interventions include padding carriers with foam to protect specimens, working with PTS manufacturers to reduce transport speeds, using tubes with clotting additives to make samples more resilient to hemolysis, and thermally insulated courier lockboxes for outpatient samples to avoid temperature-induced hemolysis and analyte degradation.
Note: New therapies involving hemoglobin-based oxygen carriers as a substitute for blood infusions in anemic patients can result in pseudohemolysis, where the serum or plasma in patients undergoing these new therapies will appear highly hemolyzed, while actual erythrocytes may not be disrupted. Ensure that you let the clinical team know that these therapies do not interfere with most analytes.
What is lipemia and how does it affect lab results?
Unlike hemolysis, lipemia is an endogenous interference, defined by an elevation of lipids in serum or plasma. It is most often detected indirectly as a measure of sample turbidity at a light absorbance of 570 nm and 660 nm, or by a cloudy pale-yellow appearance. Interference occurs spectrally in photometric assays at a similar absorbance, or by the volume displacement effect (VDE). VDE is an error by which analytes are spuriously low due to an altered ratio of aqueous to nonaqueous components in serum or plasma, which in the case of lipemia, is due to elevated nonaqueous lipids. This is most often a problem for measuring electrolytes by indirect ion-selective electrode, which involves a required dilutional step and back-calculation based on normal serum/plasma ratios.
Resolving interference caused by lipemia
Regardless of spectral or VDE interference, resolving lipemic interference can be easily accomplished by sample ultracentrifugation. In this process, the high g-forces separate the lipids and serum/plasma resulting in an interference-free specimen. Alternatively, for measuring electrolytes, direct ion-selective electrodes, found on blood gas analyzers, can be used to avoid VDE as this technique does not require a dilutional step.
Lipemia interference thresholds
Instrument and assay manufacturers often provide lipemia threshold guidelines. However, clinical laboratories should always verify the methodology that was used to create the thresholds as most manufacturers use an intravenous lipid emulsion called Intralipid® to simulate lipemia in specimens. Several studies have shown that Intralipid does not replicate the diversity of lipids found in the human body, and that consequently, intralipid-derived thresholds are often set too high to accurately detect lipemic interference.
What is icterus and how does it interfere with lab tests?
Another endogenous interference, icterus is defined by an elevation of bilirubin. It is visually identified by a yellow-green coloration of serum/plasma or detected photometrically at an absorbance of 480 nm and 505 nm. In the case of icterus, interference is either caused by spectral interference or by reactivity with assay reagents. You should note that there are two kinds of bilirubin, conjugated and unconjugated, which can each cause a different level of interference at different concentrations. The amount of each type in a given specimen can vary significantly, thus, interference for each type must be assessed separately. Manufacturers recommend that clinical labs use the lowest threshold of interference between the two.
Addressing interference caused by icterus
Unlike lipids, bilirubin cannot be readily or easily removed from serum/plasma. Resolving icterus can instead be accomplished by diluting samples to reduce icterus below the interference threshold. However, this can present issues for some assays without validated dilutional protocols or in which the lower limit of quantitation (LLOQ) is prohibitive. Consequently, laboratories should always validate manufacturer set thresholds for icteric interference, as some manufacturers may recommend a conservative universal threshold, while instrument specific thresholds may differ.
Reducing interferences to improve health care
To reduce preanalytical errors, clinical laboratories must become engaged in the processes of specimen collection, handling, and testing at all stages. Lab staff should validate manufacturer recommendations for interferences and critically analyze specimen handling systems. Additionally, labs should consider implementing protocols and tools to eliminate interferences where appropriate. By doing so, preanalytical interferences can be greatly reduced, improving the quality and reliability of lab test results and the overall function of the health care system.