Plasma Should Be Used to Measure Total Homocysteine
Use plasma, not serum, for total homocysteine measurement to ensure accurate results and avoid falsely elevated values. 1
Why Plasma Is Preferred
Sample Collection and Handling Requirements
The critical distinction between plasma and serum relates to sample processing time and accuracy:
- Plasma allows for immediate separation after blood collection, which is essential because homocysteine is continuously released from erythrocytes at room temperature 1
- Blood samples must be placed on ice immediately after collection and centrifuged within 30 minutes to prevent artifactual elevation 1, 2
- The plasma should then be frozen promptly, after which it remains stable for months 1
The Serum Problem
While serum can technically be used, it presents significant practical challenges:
- Serum homocysteine concentrations are consistently higher than plasma values in the same individuals 3
- In healthy males, mean serum homocysteine was 12.30 μmol/L versus 9.26 μmol/L in plasma (P < 0.001) 3
- In healthy females, mean serum homocysteine was 10.34 μmol/L versus 7.85 μmol/L in plasma (P < 0.001) 3
- This elevation occurs because serum requires clotting time before separation, during which homocysteine continues to be released from blood cells 4, 3
Practical Collection Protocol
To obtain accurate plasma homocysteine measurements:
- Collect blood in EDTA or heparin tubes (plasma anticoagulants) 2
- Place the tube on ice immediately after venipuncture 1, 2
- Centrifuge and separate plasma within 30 minutes of collection 1, 2
- Freeze the separated plasma right away 1, 2
- Obtain a fasting sample (8-12 hours) to avoid postprandial variation 2
Common Pitfall to Avoid
The most frequent cause of spuriously elevated homocysteine results is failure to ice the sample immediately. 2 Even a 30-60 minute delay at room temperature can markedly increase measured levels because homocysteine continues to be released from red blood cells 1, 2. This artifact affects both plasma and serum but is more problematic with serum due to the additional clotting time required.
Clinical Context
Approximately 70% of homocysteine in blood is bound to plasma proteins (primarily albumin), with only trace amounts existing as free homocysteine 1. Total homocysteine assays require a reduction step (using dithiothreitol or tri-n-butylphosphine) to release protein-bound homocysteine before measurement 1, 2. This methodology works equally well on properly collected plasma or serum, but the pre-analytical phase strongly favors plasma for accuracy 4.
Normal plasma total homocysteine values range from 5-15 μmol/L, with values >15 μmol/L defining hyperhomocysteinemia and conferring increased risk for premature vascular disease 1, 2, 4.