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Letters to the Editor |
Department of Chemical Pathology, Queensland Health, Pathology Service, Princess Alexandra Hospital, Woolloongabba, Queensland 4102, Australia
aAddress correspondence to this author at: Department of Chemical Pathology, Queensland Health Pathology Service, Princess Alexandra Hospital, Ipswich Road, Woolloongabba, Queensland 4102, Australia. Fax 61-7-3240-7070; e-mail Goce_Dimeski{at}health.qld.gov.au.
To the Editor:
Components of blood collection tubes have been implicated as interfering substances in various assays, especially immunoassays (1)(2)(3)(4)(5). Possible interfering components for blood collection tubes include the lubricants, clot activators, surfactants, and barrier gels. In 1 study, a surfactant was identified as the interferent (1). To our knowledge, no problems have been reported with general chemistry assays. Here we describe an apparent contamination of Terumo tubes with magnesium.
In an evaluation of collection tubes, we collected samples from volunteers via a Vacutainer system into 3 tubes (Greiner, BD, or Terumo). The samples were obtained with no conscious order of draw, and the 3 types of tubes were treated identically. The fill volume of drawn blood was 5–7 mL for all tubes. The samples were stored at room temperature (
21 °C) for <45 min before centrifugation. Magnesium was measured by a xylidyl blue method on a Modular analyzer (Roche). The mean magnesium concentrations were higher (P <0.001) by nearly 0.1 mmol/L for plasma and serum from the Terumo VENOSAFE gel-containing blood collection tubes than for samples from the corresponding BD and Greiner tubes (Table 1
).
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To identify the source of the increased magnesium results, we added 0.5 mL of normal saline to 5 separate tubes of each of the types listed in Table 1
. The tubes were inverted several times to wet the inner tube walls and rubber stopper, and magnesium was then measured in the saline. The measured magnesium concentrations (Roche Modular, xylidyl blue method for all samples) were <0.05 mmol/L in saline from the Greiner and BD tubes but were [mean (SD)] 0.58 (0.04) mmol/L in the Terumo lithium heparin tubes and 0.60 (0.06) mmol/L in the Terumo serum tubes. In a separate experiment, 5 lithium heparin and 5 serum tubes of the Terumo tubes listed above were fully filled with 7 mL of saline (rather than 0.5 mL); the mean (SD) magnesium results were 0.05 (0.00) and 0.06 (0.00) mmol/L, respectively. The results suggested that the magnesium or an interfering substance was being released from the tube components (gel, rubber stopper, or other components within the tubes). When the 0.5-mL saline samples from the Terumo tubes were measured on a Vitros 950 (Ortho Clinical Diagnostics) analyzer by a formazan dye method, the results were similar: 1.00 (0.04) and 1.00 (0.07) mmol/L for serum and plasma, respectively.
In contrast to the results with gel-containing tubes, saline (0.5 mL) added to 2 types of Terumo tubes without gel had low mean (SD) magnesium concentrations: 0.11 (0.02) mmol/L for 5.5-mL VF-076SHL tubes (lot no. 0409007) and 0.06 (0.01) mmol/L 5.5-mL VF-076SP tubes (lot no. 0406061). The measured concentration was higher, 0.18 (0.01) mmol/L, for a lithium-heparin tube with gel (3.5 mL; VF-054SAHL; lot no. 0501041). The data suggest a role of the gel or other components in the gel tubes that may not be present in the non-gel tubes (or are present at different concentrations).
To investigate whether the tubes contained magnesium or a substance that interfered in the colorimetric magnesium methods, we used atomic emission spectrometry. We added 0.5 mL of saline to 3 lithium-heparin (VF-108SAHL) and 3 serum (VF-108SAS) Terumo tubes. The tubes were allowed to mix for 10 min on a roller to wet the inner tube walls and rubber stopper. The magnesium in the saline was then measured on the Roche Modular systems and by atomic emission (Varian Vista AX ICP Spectrometer). The mean (SD) results for heparin and plain tubes, respectively, were 1.00 (0.07) and 1.13 (0.10) mmol/L on the Roche Modular system and 1.03 (0.07) and 1.17 (0.12) mmol/L on the Varian Vista AX system.
The only previously reported blood tube–related problem with magnesium analysis has been an apparent positive interference in measurements of free (ionized) magnesium in samples collected in BD Vacutainer plastic tubes (serial no 367820) (6). The authors concluded that the positive bias was likely attributable to interference by silicone in the stopper lubricant and the clot activator.
In summary, higher concentrations of magnesium are found in serum and plasma from blood collected in Terumo tubes, especially those containing gel. The data suggest that the tubes contain magnesium and not a substance that interferes in colorimetric assays.
References
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