Procedures
Accurate lab testing for heme derivatives is essential for diagnosing and managing various conditions. Understanding the underlying principles, the importance of proper technique, and potential pitfalls can greatly enhance the reliability of results
Test Principles
- Bilirubin: Diazo reaction with Ehrlich’s reagent measures total and direct bilirubin. Jendrassik-Grof method minimizes interferences
- Urine Bilirubin: Dipstick with diazo reagent detects conjugated bilirubin
- Urine Urobilinogen: Ehrlich’s reagent detects urobilinogen. Quantitative methods offer better accuracy
- Iron Studies: Colorimetric assays measure serum iron, TIBC, and transferrin saturation. Immunochemical assays quantify ferritin
- Porphyrin Analysis: Chromatography (HPLC) separates and quantifies porphyrins, detected by fluorescence or spectrophotometry
- Fecal Occult Blood Test (FOBT): Guaiac-based tests detect heme’s peroxidase activity. Immunochemical tests (FIT) use antibodies for human hemoglobin
Special Precautions
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Specimen Collection
- Bilirubin: Protect from light
- Urine Urobilinogen: Use preservative, protect from light
- Iron Studies: Avoid hemolysis, collect before therapy/transfusion
- Porphyrins: Protect from light, use appropriate containers/preservatives
- FOBT: Multiple samples, follow dietary/medication restrictions
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Specimen Processing
- Timely processing is key
- Appropriate centrifugation
- Proper storage (refrigeration or freezing)
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Analytical
- Reagent Quality: Use fresh, properly stored reagents
- Calibration: Regular calibration with standards
- Quality Control: Run QC samples regularly
Interfering Substances
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Common Interferences
- Lipemia, hemolysis, medications, ascorbic acid, porphyrinogens
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Mitigation
- Use appropriate techniques to clear lipemia
- Avoid hemolysis
- Be aware of patient’s medications
Troubleshooting
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General Steps
- Review patient preparation
- Check collection and processing
- Evaluate reagent quality
- Consider interferences
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Specific Issues
- Unexpected Bilirubin: Light exposure, delayed processing
- Unexpected Iron Studies: Hemolysis, recent iron therapy
- Inconsistent Porphyrin Analysis: Extraction/separation issues
- False Positive/Negative FOBT: Dietary or medication factors