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Letters to the Editor |
1 Canisius-Wilhelmina Medical Centre, Department of Clinical Chemistry, Nijmegen, The Netherlands
2 BD Diagnostics, Preanalytical Systems, Plymouth, United Kingdom
aAddress correspondence to this author at: Canisius-Wilhelmina Medical Centre, Department of Clinical Chemistry, Weg door Jonkerbos 100, 6500 GS Nijmegen, The Netherlands. Fax 31-24-3658671; e-mail j.v.d.ouweland{at}cwz.nl.
To the Editor:
Many laboratories perform routine chemistry analysis with serum or plasma–based blood collection tubes containing separator gels. A barrier polymer is present at the bottom of the tube. The density of the material causes it to move upwards during centrifugation to the supernatant–cell interface, where it forms a barrier separating plasma or serum from cells. Supernatant plasma or serum may be aspirated directly from the collection tube, eliminating the need for transfer to a secondary tube.
We recently observed 2 occasions within 1 month when both the ion-selective electrode and chemistry sampling probes of the analyzer (Modular Analytics, Roche Diagnostics) were occluded. In both cases, the occlusion was caused by inappropriate gel barrier formation after centrifugation (2000g for 10 min at room temperature) of the primary tubes. Plasma (BD Vacutainer® PSTTM II) and serum (BD Vacutainer® SSTTM II) samples had been collected from 2 patients diagnosed with multiple myeloma. In the plasma tube, the gel barrier material was floating on the surface of the supernatant, and in the serum tube the gel barrier was entwined with the serum and erythrocytes (Fig. 1
). Analysis of blood samples from both patients in plain serum tubes showed highly increased total protein concentrations (139 and 142 g/L; reference interval 60–80 g/L) caused by the presence of a monoclonal-protein (an IgG-
of 89 g/L and an IgA- of 92 g/L, respectively). Furthermore, plasma viscosity values were 5.7 and 7.1 centipoise, respectively, (reference interval 1.5–2.0 centipoise) and specific gravities, as measured by weighing 500 µL of plasma or serum, were 1.037 and 1.039, respectively.
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The positioning of the gel in the tube is influenced by a number of variables, some of which are controlled by the tube manufacturer (specific gravity, yield stress, viscosity, density, and tube material), some by the hospital laboratory (centrifugation speed, temperature, acceleration and deceleration conditions, and storage conditions), and some of which are patient specific [heparin therapy, low hematocrits, increased plasma proteins (1), and the use of iodinated blood contrast media(2)].
A retrospective analysis in our clinical chemistry laboratory of total protein requests showed that during a 5.5-year period, 5 of 13 221 patients (0.04%) had a total protein concentration >135 g/L. Therefore, we anticipate that our laboratory should observe this phenomenon several times a year. This number may vary depending on the degree of oncology-related patients visiting the hospital. To our knowledge, only one single case report has been published on this topic for a blood collection system from a different manufacturer (1).
Laboratories, in which preanalytical steps include automatic centrifugation and sample transport to on-line chemistry analyzers, are particularly vulnerable for occlusion of sample probes from inappropriately separated blood samples. Visual checks to determine the adequacy of barrier formation after centrifugation should prevent the inappropriately separated samples from being transferred to the analyzer, although labels on tubes can often prevent rapid visual inspection.
Despite the fact that inappropriate barrier formation is occurring at a low frequency, the impact on costs (sample probe replacement and down-time of the analyzer causing discontinuation of the workflow process) and patient outcomes (e.g., potential danger of reporting falsely low results when no sample is aspirated) can be substantial. Laboratories and tube manufacturers should be aware of the limitation of using any tubes containing gel-separator in patients with high plasma viscosity because of the presence of high total protein concentrations. In these particular cases, subsequent blood drawings should be collected in non–separator-based blood collection tubes. We will conduct further with the tube manufacturer to assess the amount of total protein at which gel barrier formation is compromised. Our observation contributes to the increasing awareness of the impact on patient outcomes and the costs of laboratory errors occurring in the preanalytical phase (3).
References
The following articles in journals at HighWire Press have cited this article:
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  M. Fatas, P. Franquelo, and R. Franquelo Anomalous Flotation of Separator Gel: Density or Viscosity? Clin. Chem., April 1, 2008; 54(4): 771 - 772. [Full Text] [PDF]
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