Cloudy Serum in SST Tubes: Clinical Significance
Cloudy or opaque serum in an SST tube indicates lipemia (elevated lipid content in the blood), and such samples should be discarded or reported as compromised, as they can interfere with laboratory analysis and produce inaccurate results. 1
Primary Causes of Cloudy Serum
- Lipemia (most common): Elevated triglycerides and lipoproteins create a milky or turbid appearance in serum, typically occurring in non-fasting patients or those with hypertriglyceridemia 1
- Insufficient clotting: Residual fibrin strands can create a cloudy appearance and may clog analyzing instruments, often resulting from inadequate clotting time before centrifugation 2
- Hemolysis: While typically causing an orange or red discoloration rather than cloudiness, severe hemolysis can contribute to serum opacity 1
Quality Control Standards
The Journal of Extracellular Vesicles guidelines explicitly state that plasma or serum samples appearing opaque (fat), orange, or red (hemolysis), or containing visible clumps or insoluble strands should be discarded or reported as compromised. 1 This recommendation applies across clinical laboratory settings to ensure accurate test results.
Impact on Laboratory Testing
- Lipemic samples interfere with spectrophotometric measurements by scattering light, leading to falsely elevated or decreased values depending on the assay methodology 1
- Cloudy serum from insufficient clotting can clog automated analyzers and produce spurious results due to fibrin interference 2
- The gel separator in SST tubes is designed to prevent contamination with cellular components, but cannot compensate for pre-analytical issues like lipemia 2
Prevention Strategies
- Overnight fasting reduces lipoprotein concentrations and minimizes lipemia in routine blood collection 1
- Ensure proper clotting time: BD Diagnostics recommends 30 minutes for SST tubes before centrifugation to prevent fibrin formation 2
- Follow proper centrifugation protocols at 1500-2000 × g for 10 minutes to ensure adequate serum separation 3
- Process specimens within 2 hours of collection to prevent cellular degradation and maintain sample integrity 2, 3
Clinical Action Required
When encountering cloudy serum, the specimen should be rejected and recollection requested with proper patient preparation (fasting if appropriate) to obtain a valid sample for accurate laboratory analysis 1