The order of draw – myth or science?
National and international guidelines, such as those issued by the Clinical Laboratory Standards Institute (CLSI) or the World Health Organization (WHO), recommend that an order of tubes should be followed during blood drawing, i.e., blood culture/sterile tubes first, followed by plain tubes/gel tubes, and then tubes containing additives. This specific strategy is aimed to prevent contamination of sample tubes with additives from previous tubes, such as sodium citrate or more commonly potassium-EDTA (K-EDTA).
These recommendations are mostly based on a case report published by Sun in 1977, and a follow-up study by Calam and Cooper in 1982, which reported that an incorrect order of draw caused hyperkalemia and hypocalcemia, two surrogate markers of in vitro K-EDTA sample contamination.
The authors did acknowledge, however, that contamination with additives only occurred during difficult venipunctures and could not be replicated under ideal phlebotomy conditions. It has been definitively demonstrated, by measuring EDTA, that reverse order of draw of blood samples using closed loop phlebotomy systems does not cause EDTA contamination. This has been subsequently confirmed in another study.
Although it seems difficult to reconcile the conflicting results emerged from different studies, it may be that a random order of draw using poor sample collection techniques and/or during difficult venipunctures may result in cross-contamination of sample tubes, thus ultimately jeopardizing the quality of testing.
This idea is supported by a study of Berg et al., which showed that only 6% of blood collections were performed using the conventional manufacturer prescribed closed loop system in a major emergency department in the UK.
Lima-Oliveira et al. also recently described a patient case in which deviation from the standard blood sampling procedure and recommended order of draw resulted in sample EDTA contamination with subsequent increase in potassium and decrease in calcium concentration.
In general, a significant bias may be typically observed in the serum values of calcium, chloride, lactate dehydrogenase (LDH), magnesium and potassium starting from 5% contamination with K-EDTA blood, whereas the serum values of sodium, phosphate and iron may be biased starting from 29% contamination with K-EDTA blood.
It has been earlier shown that contamination with EDTA (and, to a lesser extent, with sodium citrate) is still relatively frequent and may be difficult to identify. As this is not probably due to the use of a random order of draw of blood samples in a closed loop system, it seems plausible that in vitro K-EDTA and citrate contamination may occur with open blood collection systems by syringe needle or syringe tip contamination when delivering collected blood into K-EDTA or citrate sample tubes before other tubes, and by direct transfer of blood from K-EDTA or citrate containing tubes to other sample tubes.
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The latter circumstance can be easily detected by the laboratory staff, because it would generate gross abnormalities in surrogate markers. Nevertheless, more subtle contamination is possible with the former condition, which is less easily identifiable using these markers and may also cause misdiagnosis and/or mismanagement of patients
In summary,
1) There is currently not enough evidence to support the recommended download order (if a closed loop is used);
2) There is no evidence to confirm that the agreed sequence helps prevent cross-contamination;
3) Epidemic patterns are rare and further research is needed to determine the mechanisms of infectious patterns so that appropriate prevention plans can be made.
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