Separator Gel in Blood Collection Tubes Exhibit Anomalies

Clinical samples that have densities greater than that of separator gel in specimen tubes may exhibit gel flotation to the top of the specimen upon centrifugation.

Inappropriate separator gel flotation can occur in specimens with high protein content and for automated analytical systems this can lead to mechanical disruption and potential inaccurate result reporting upon aspiration into instrument sampling probes.

At the University of Arkansas for Medical Sciences (Little Rock, AR, USA), scientists have determined the relative specimen densities and estimated total protein contents at which specimen gel flotation would occur upon centrifugation in commonly used commercial specimen tubes. They initiated a comparative study of separator gel density using prepared dextran solutions.

In order to investigate the impact of different specimen tubes on the frequency of gel flotation in the clinical laboratory, a survey was undertaken of all total protein results from 46, 808 routine clinical samples, spanning a one year period. Eleven serum samples in the previous year total protein-result database exhibited an estimated total protein content of greater than 14.06 g/dL and they thus were deemed to be at risk for gel flotation in specific serum specimen tubes.

Specific gravity of several dextran solutions was determined by direct hydrometry. The dextran solutions were introduced to serum and plasma lithium heparin BD Vacutainer (Becton, Dickinson and Company; Franklin Lakes, NJ, USA) specimen tubes and into Vacuette specimen tubes (Greiner Bio-One; Frickenhausen, Germany), containing separator gel. Following centrifugation, the specimen tubes were examined for gel flotation. The total protein content in clinical samples that would result in gel flotation was estimated for different specimen container types. Flotation was observed at a lower dextran solution density for Bio-One Vacuette Lithium-Heparin tubes than for the BD Lithium-Heparin Plasma Vacutainer SST tubes in both serum and plasma separator gel-specimen tubes.

Some differences were observed between lots of the same specimen tube types. In serum tubes, the range of specific gravity where gel flotation was observed was 1.054–1.057 for BD Vacutainers and 1.044–1.047 for Greiner Vacuette tubes. For the Lithium-heparin plasma tubes the results were 1.054–1.057 for Vacutainers and 1.047–1.048 for Vacuettes. The total protein content of the clinical sample described in the study for which gel flotation was observed in a Greiner plasma specimen but not a BD plasma specimen tube was 14.4 g/dL. No samples exhibited protein contents higher than 16.12 g/dL, an estimated total protein content that would result in gel flotation in both BD and Greiner serum specimen tubes. The study was published in September 2011 in the Archives of Pathology and Laboratory Medicine.

Related Links:
University of Arkansas for Medical Sciences
Becton, Dickinson and Company
Greiner Bio-One