Why is the lavender tube filled first before red when using syringe method of vein puncture?

When using a syringe method for vein puncture, purple-stoppered (EDTA) and red-stoppered (clot activator) tubes are usually filled first before the red ones. This has several reasons:

1. Avoiding the First Pass Effect: The "first-pass effect" occurs when the needle punctures the vein, and the initial rush of blood may dilute or hemolyze the sample, making it unsuitable for analysis. By collecting the EDTA tube first, the initial diluted or hemolyzed blood is discarded, ensuring the subsequent tubes contain better quality blood.

2. Additive Interference: The clot activator present in the red-stoppered tubes can potentially interfere with some laboratory tests, such as those measuring electrolytes, glucose, or other analytes. By collecting the EDTA tube first, we avoid the potential for additive contamination and ensure accurate test results.

3. Stability of Samples: The anticoagulant present in the EDTA tube, EDTA (ethylenediaminetetraacetic acid), stabilizes the blood and prevents clotting. This is crucial for tests that require a longer time to process or transport, such as those involving hematology or specialized chemistry panels. Red-stoppered tubes, on the other hand, are primarily used for tests that require serum, which is the liquid portion obtained after clotting.

While the sequence of tube collection (purple, then red) is often the preferred practice, some laboratories or healthcare providers may have their own specific protocols based on the tests being ordered and their internal policies. It's always best to follow the recommended order of tube filling to ensure the accuracy and reliability of the blood samples collected.