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Letters |
Laboratory Medicine, University Hospitals Leuven, Katholieke Universiteit Leuven, Herestraat 49, B-3000 Leuven, Belgium
aAddress correspondence to this author at: Laboratory Medicine, Immunology, University Hospitals Leuven, Herestraat 49, B-3000 Leuven, Belgium. Fax 32-16-34-70-42; e-mail xavier.bossuyt{at}uz.kuleuven.ac.be.
To the Editor:
Capillary zone electrophoresis (CZE) using fused-silica capillaries has become a well-accepted method for the separation of serum proteins and for the detection of monoclonal components in human serum (1)(2)(3)(4). In earlier methods, such as those that use agarose gels, quantification of the protein fractions was based on dye binding, whereas CZE uses ultraviolet detection at 214 nm for direct protein quantification via the peptide bonds. Any substance or drug that is present in serum and that absorbs at 214 nm potentially can interfere with CZE analysis. Few interfering substances have been reported. Radiocontrast media that absorb at 214 nm interfere with CZE and can simulate a monoclonal component (5)(6)(7), and the antibiotic piperacillin-tazobactam (Tazocin®; Wyeth Lederle) produces a small peak in the ß-globulin fraction (8). In the present report, we describe that the sulfamide sulfamethoxazole produces a small peak at the anodal site of the albumin fraction.
Shown in panels A and B of Fig. 1
are CZE electropherograms (Beckman Coulter Paragon CZE 2000, software Ver. 1.6) of two samples obtained from patients who received intravenous sulfamethoxazole-trimethoprime (400 mg of sulfamethoxazole/80 mg of trimethoprime, 12 ampoules/day for 6 days). In the sample shown in Fig. 1B
, there was a small monoclonal protein in the gamma region. In each case, a distinct peak was observed at the anodal site of the albumin fraction. No such peak is present in a typical CZE electropherogram, and none was seen in the CZE electropherograms of specimens from the same patients as in Fig. 1
, A and B, collected 2 or 3 days after sulfamethoxazole-trimethoprime administration. After this time period, the antibiotic has been cleared from the blood stream. In patients with normal kidney function, the elimination half-life of sulfamethoxazole is 9 h and that of trimethoprime is 10 h. Protein binding for sulfamethoxazole is 66%, whereas for trimethoprime, it is 42–46%.
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When we added sulfamethoxazole but not trimethoprime to a normal serum sample, an abnormal peak appeared in the CZE electropherogram (Fig. 1C
) in the same region as the extra peak observed in the electropherograms from patients receiving the antibiotic. The addition of various concentrations of sulfamethoxazole (final concentrations of 480, 240, 120, 60, 30, 15, 7.5, and 3.75 mg/L) to a normal serum sample led to the appearance of a dose-dependent peak. The peak was largest at a sulfamethoxazole concentration of 480 mg/L, with a gradual reduction of the size of the peak with decreasing sulfamethoxazole concentrations. No interference was observed with sulfamethoxazole concentrations 
7.5 mg/L.
We observed a peak similar to the abnormal peaks in panels A and B of Fig. 1
in 10 other patients. All of these patients had received sulfamethoxazole. The position of the peak was the same for all patients. The size of the peak, however, varied slightly. Oral administration of sulfamethoxazole-trimethoprime produced the appearance of the abnormal peak on CZE. Removal of nonprotein components by use of D-SaltTM Dextran Plastic Desalting Columns (Pierce) (7) removed the sulfamethoxazole peak.
In summary, sulfamethoxazole produces a small peak at the anodal site of the albumin fraction with CZE. Medical technologists, clinical pathologists, and clinicians should be aware of this interference, which is not seen with classic agarose gel electrophoresis of serum proteins.
References
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