ESR Performed on Old Blood Samples: Falsely Elevated Results
When an ESR test is performed on a blood sample that is too old, the result will be falsely elevated (inaccurately high). This occurs because aged blood samples undergo changes that artificially increase the sedimentation rate, making the test unreliable for clinical decision-making.
Mechanism of False Elevation
The erythrocyte sedimentation rate measures how quickly red blood cells settle in a vertical tube, which reflects blood viscosity and serves as a surrogate marker of inflammation 1. When blood samples age beyond the recommended testing window, several physiological changes occur:
- Red blood cell morphology deteriorates over time, causing cells to aggregate more readily and settle faster than they would in fresh samples 2
- Temperature fluctuations during storage affect the accuracy of ESR measurements, as the test is sensitive to environmental conditions 2
- Plasma protein changes occur in stored samples, altering the normal sedimentation dynamics 2
Critical Timing Requirements
- ESR testing requires at least 2 hours using traditional Westergren methodology, but the blood sample itself must be fresh 2
- The test should ideally be performed within 2-4 hours of collection to ensure accuracy, as delays beyond this window introduce measurement errors 2
- Rapid alternative methods (micro ESR) can provide results in as little as 20 minutes with strong correlation to conventional methods (r = 0.987), reducing the risk of sample aging 3
Clinical Implications and Pitfalls
The most important caveat is that falsely elevated ESR from old samples can lead to unnecessary diagnostic workup for inflammatory conditions that don't exist 4, 5. This is particularly problematic because:
- ESR values >100 mm/hr have 90% predictive value for serious underlying disease (infection, collagen vascular disease, or metastatic tumor), so a false elevation could trigger extensive and costly investigations 4
- In conditions like giant cell arteritis, where ESR >40 mm/h has 93.2% sensitivity, a falsely elevated result from an old sample could lead to inappropriate urgent referrals and treatment 1
- The test already lacks optimal sensitivity and specificity even when performed correctly, so any additional source of error compounds its limitations 4, 5
Practical Recommendations
- Always verify sample age before interpreting ESR results—if the sample is more than 4 hours old, request a fresh specimen rather than relying on potentially misleading data 2
- Consider rapid ESR methods (micro ESR using capillary tubes) that can be completed in 20 minutes, minimizing the window for sample degradation 3
- If an unexpectedly elevated ESR is encountered without clear clinical correlation, repeat the test with a fresh sample in several weeks rather than pursuing exhaustive diagnostic workup 4, 5
- Use EDTA-anticoagulated samples following ICSH recommendations, as these provide more standardized results than citrated specimens 6