Unexpected abnormal flotation of gel separator in tube of post dialysis samples: a case report highlighting the critical role of sampling site selection.

Proper preanalytical handling of blood samples is critical to ensure the reliability of laboratory results, particularly in patients undergoing hemodialysis, where biochemical monitoring is essential for assessing dialysis adequacy and guiding treatment decision. We reported three cases of abnormal post-dialysis gel separator flotation in blood collection tubes from patients undergoing hemodiafiltration: in each case, the gel migrated to the top of the tube, with plasma trapped below and blood cells remaining at the bottom. Marked hyperproteinemia and hypercalcemia were observed in the plasma, inconsistent with the patient's clinical status and pre-dialysis values. These findings raised suspicion of a preanalytical error potentially associated with the hemodialysis procedure. On-site investigations conducted in collaboration with the dialysis center for four additional patients, combined with a better understanding of the principles of hemodiafiltration and the potential sampling sites, confirmed that the gel migration anomaly was secondary to unsuitable sampling from the venous line (outflow line) of the dialysis circuit instead of the arterial one (inflow line). In conclusion, we highlighted the critical role of adhering to the appropriate sampling site when performing post-dialysis blood tests: the arterial line was identified as the appropriate site for post-dialysis blood sampling, while the venous line should be reserved exclusively for infusion or reinjection purposes and must never be used for blood collection at the end of dialysis.