Laboratory test results provide information for diagnosing conditions, monitoring health, and guiding treatment. They are generated through measurements of substances in blood, urine, or other body fluids. Accurate interpretations rely on sample integrity. Various factors can influence reliability, potentially leading to misleading results. One such factor is lipemia, which can significantly affect the accuracy of many diagnostic tests, requiring careful consideration by professionals.
Understanding Lipemia
Lipemia describes the presence of excessive lipids (fats) in a blood sample. Normally, the liquid portion of blood (serum or plasma) appears clear or pale yellow. A lipemic sample, however, appears cloudy, milky, or turbid. This cloudiness results from high concentrations of fat particles, primarily chylomicrons and very-low-density lipoproteins (VLDL). These particles scatter light, causing the opaque look.
Factors Contributing to Lipemia
Several factors can cause lipemia. The most common cause is temporary, physiological lipemia from recent high-fat meal consumption. Patients are often instructed to fast for 8-12 hours before blood collection to prevent this.
Beyond diet, lipemia can also stem from medical conditions or treatments. Pathological causes include metabolic disorders like hyperlipidemia and uncontrolled diabetes. Liver disease, kidney disease, and acute pancreatitis can also elevate blood lipids. Additionally, some medications, such as intravenous lipid emulsions (parenteral nutrition) or propofol, can contribute to significant lipemia.
Mechanisms of Interference
Lipemia interferes with lab tests through several mechanisms. The most frequent involves light scattering, where abundant lipid particles cause turbidity. Many lab tests use spectrophotometric methods, which measure how much light a sample absorbs or transmits to quantify substances. The cloudy appearance of lipemic samples scatters light, leading to inaccurate light readings and falsely elevated or decreased results, depending on the assay.
Another significant mechanism is volume displacement. High lipid concentrations physically reduce the proportion of the aqueous (watery) phase in the plasma. Since many analytes, particularly electrolytes, are dissolved in the water portion of the blood, this displacement leads to an artificially lower concentration of these substances when measured by methods that assess the total sample volume. This effect is especially relevant for indirect ion-selective electrode (ISE) measurements, which are common for electrolyte analysis.
Furthermore, lipids can sometimes chemically interact directly with the reagents used in laboratory assays. These interactions can alter the intended chemical reactions, thereby affecting the accuracy of the measurement and potentially leading to either false increases or decreases in analyte levels.
Commonly Affected Laboratory Tests
Lipemia can impact many common lab tests, leading to misleading results. Liver function tests (LFTs) like bilirubin, aspartate aminotransferase (AST), and alanine aminotransferase (ALT) can show interference. Bilirubin may be falsely elevated, while AST and ALT can exhibit negative or positive interference depending on lipemia severity.
Kidney function tests, including creatinine and urea, are also susceptible. Electrolytes like sodium, potassium, and chloride often show falsely decreased values due to volume displacement, especially in severely lipemic samples. Glucose measurements can also be affected by light scattering. Other tests showing inaccuracies include enzymes like amylase, phosphorus, calcium, total protein, albumin, and uric acid, which generally show positive interference.
Managing Lipemia for Accurate Results
Addressing lipemia is important for ensuring the accuracy of lab test results. Proper patient preparation is a primary step, often involving fasting for 8-12 hours before blood collection. This fasting significantly reduces diet-induced lipemia.
In the laboratory, samples are visually inspected for turbidity, and many modern analyzers also employ automated detection methods, such as a lipemic index, to quantify the degree of lipemia. If a sample is significantly lipemic, laboratories can use several techniques to mitigate interference. These include sample dilution or specialized processes like ultracentrifugation, which is considered a reliable method for removing lipid particles. High-speed centrifugation can also be used to clear some lipid types.
For electrolytes, direct ion-selective electrode (ISE) methods, which measure analytes in the aqueous phase without dilution, are less affected by lipemia. However, if lipemia is severe and cannot be effectively managed by these methods, collecting a new sample after proper patient fasting is often the most appropriate course of action to obtain accurate results.