Is it possible to transfer blood from a Serum Separator Tube (SST) to an Ethylenediaminetetraacetic acid (EDTA) tube for running a Hemoglobin A1c (A1c) test?

Transferring Blood from SST to EDTA Tube for HbA1c Testing

Transferring blood from a Serum Separator Tube (SST) to an EDTA tube for HbA1c testing is not recommended as it can lead to inaccurate results and potentially impact patient care decisions.

Why This Practice Should Be Avoided

Blood Collection Fundamentals

• Blood collection tubes contain specific additives designed for particular tests 1
• SST tubes contain clot activators and gel separators designed to separate serum from cells after clotting 1
• EDTA tubes contain anticoagulants that prevent clotting by chelating calcium 2

Specific Issues with SST to EDTA Transfer

1. Clotting Process Interference:

   • SST tubes are designed to promote clotting (30-60 minutes clotting time recommended) 1
   • Once blood has begun to clot in an SST tube, transferring to EDTA is ineffective as the anticoagulant cannot reverse clotting 1
   • Microclots may form and interfere with accurate HbA1c measurement 1

2. Additive Contamination:

   • Cross-contamination between tube additives can occur during transfer 1
   • The Clinical and Laboratory Standards Institute recommends a specific blood draw order to avoid carry-over of additives 1
   • Gel separators in SST tubes can absorb blood constituents and interfere with clinical tests 1

3. Sample Integrity Issues:

   • HbA1c testing requires intact red blood cells with proper anticoagulation 1
   • Proper tube filling is critical - underfilled EDTA tubes (<50% filled) show significant bias in HbA1c results 3
   • Transferring blood after it has been in an SST tube compromises sample integrity 1

Proper Collection for HbA1c Testing

For accurate HbA1c testing:

• Collect blood directly into EDTA tubes (lavender top) 1
• Ensure proper tube filling (at least 50% of recommended volume) 3
• Gently invert the tube immediately after collection (8-10 times for EDTA tubes) to ensure proper mixing of blood with anticoagulant 1
• Process samples according to laboratory protocols - whole blood samples are generally stable for up to 1 week at 4°C 1

Clinical Implications

Inaccurate HbA1c results can lead to:

• Misdiagnosis of diabetes or prediabetes 1
• Inappropriate treatment decisions 1
• Failure to identify patients at risk for complications 1

Alternative Approaches

If HbA1c testing is needed but proper EDTA collection is not available:

• Collect a new sample in the appropriate EDTA tube 1
• Consider alternative glycemic markers such as fructosamine or glycated albumin if red blood cell abnormalities are present 1
• Use plasma glucose testing (fasting or 2-hour post-load) as an alternative diagnostic approach 1

Remember that HbA1c is a critical test for diabetes diagnosis and management, with specific cut-points (≥6.5% for diagnosis) that require accurate measurement 1. Compromising sample integrity through improper collection or tube transfer can directly impact patient care.