Q: Is fasting prior to Hcy testing required or is a morning sample necessary?
A: No, but avoid heavy meals 6–12 h before sampling.
Q: Posture during blood collection?
A: Not important; slightly lower in supine position.
Q: Duration of stasis?
A: Not important.
Q: Blood collection tube?
A: Heparin plasma preferred SST tube for serum if possible.
Q: Use of Hcy stabilizers ?
A: Debated, do not use 3-deazaadenosine.
Q: Stability of tHcy in whole blood? A: Increases 10% per hour at room temperature; keep cool until centrifugation, or centrifuge within 30–60 min.Q: Stability of tHcy in plasma/serum after removal of blood cells?
A: Stable for at least 4 days at 20°C, for several weeks at 4°C, and several years at -20°C.
Q: Does mild/moderate hemolysis change the measured tHcy level?
A: No, but it may interfere with some methods.
Q: What is a significant change between tHcy measured in blood collected on two separate occasions?
A: 20 % (depends on method performance).
Q: How often is it necessary to check a normal tHcy level?
A: Consult with your doctor.
Q: When is repeat testing necessary to check vitamin response?
A: 14 days and 3 months after the start of treatment, then every 1–3 years.
Q: How are borderline tHcy levels treated?
A: Asymptomatic subjects: new test in 6–12 months; give advice on lifestyle changes.
Q: Are there international tHcy calibrators?
A: No, currently, no universally certified tHcy standard is available.
Q: Can tHcy measured in two different laboratories be compared?
A: Only if the same calibrators are used.
Q: What analytic range is required for a tHcy test method?
A: The method should cover the 0.5th-95th percentiles in the general population (~3-40 μmol/L).
* Q: Is the tHcy test interfered with by plasma cystathionine?
A: Yes, some enzymatic methods are interfered with by endogenous cystathionine.
* Q: How critical is cystathionine interference to patient tHcy levels?
A: It is critical since millions of renal failure patients have significantly elevated plasma cystathionine levels, and method interfered by cystathionine will falsely report significantly higher (20-300%) tHcy levels.
Q: What if the tHcy level is beyond the analytic range?
A: Dilute the serum sample with water.