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Monoclonal paraprotein may interfere with the Roche direct HDL-C plus assay.

To the Editor:

We report here the observation of a falsely decreased HDL-cholesterol (HDL-C) result obtained with the Roche HDL-C Plus assay (polyethylene glycol-modified enzymes, sulfated [alpha]-cyclodextrin, and dextran sulfate; cat no. 1930648) in an asymptomatic 78-year-old woman (1). The total cholesterol of the patient was 3.2 mmol/L, the triglycerides were 1.3 mmol/L, and the HDL-C was suppressed (<0) on a Roche Modular analyzer. The same analysis was repeated on the Johnson & Johnson Vitros (with magnetic separation of particles; cat. no. 1042523) and the Beckman LX-20 (with a polyanionpolymer/ detergent reagent; cat. no. 650207) analyzers. The HDL-C concentrations according to these methods were 1.2 and 1.0 mmol/L, respectively. On the Roche analyzer, the absorbance of the blank was higher than the final absorbance result, giving a negative value by subtraction. Lipoprotein electrophoresis revealed a normal [alpha] band.

We therefore suspected the presence of a paraprotein that would precipitate with the first reagent, possibly related to dextran. Serum total proteins and albumin were 101 and 40 g/L, respectively. Serum protein electrophoresis revealed a monoclonal band in the [gamma] region that was unknown to the treating physician. This monoclonal band was characterized by immunofixation as an IgM-[kappa] band that was estimated at 30 g/L.

We selected other sera from patients known to have a monoclonal band and assayed these sera for HDL-C with two different assays: the Roche HDL-C Plus assay and a precipitation method using heparinmanganese (Table 1).

The interference was identified by visual inspection of the absorbance graphs and confirmed with the precipitation method. We identified other cases of falsely decreased results in some samples, independent of the concentration or type of paraprotein involved (Table 1). The exact incidence of this bias was not known, but it was possibly not uncommon because we found several cases in a small series of patients.

Such interference does not always lead to suppression of a HDL-C result because it could simply lower the result. This decrease would go unnoticed with the current Roche application. In our view, some modifications could be introduced to screen for this phenomenon. This interference by monoclonal bands with the HDL-C direct assay is not reported in the 1997 edition of Effects of Disease on Clinical Laboratory Tests by Friedman and Young (2) or in the manufacturer's package insert, except for the following statement: "In rare cases, increased immunoglobulin concentrations can lead to falsely increased HDL-cholesterol results". There is no mention of falsely decreased results in patients with paraproteins. Because the prevalence of paraproteins in the population is significant, it is conceivable that some patients with benign paraproteins may have a falsely decreased HDL-C result that could alter their cardiovascular risk estimates and treatment by physicians.

References

(1.) Warnick G, Nauck M, Rifai N. Evolution of methods for measurement of HDL-cholesterol: from ultracentrifugation to homogeneous assays. Clin Chem 2001;47:1579-96.

(2.) Friedman RB, Young DS. Effects of disease on clinical laboratory tests. Washington: AACC Press, 1997:3-225-3-229.

Nadine Kadri [1] Pierre Douville [1]* Pierre Lachance [2]

[1] Service de Biochimie CHUQ-Hotel-Dieu de Quebec 11 Cote du Palais Quebec, G1R 1P5 Canada

[2] Service de Biochimie Hotel-Dieu de Levis 143 rue Wolfe Quebec, G6V 3Z1 Canada

* Address correspondence to this author at: Service de Biochimie, CHUQ-Hotel-Dieu de Quebec, 11 Cote du Palais, Quebec, G1R 1P5 Canada. Fax 418-691-5709; e-mail pierre.douville@bcx.ulaval.ca.
Table 1. cTnT results obtained by the CARDIAC T Quantitative Rapid
and Troponin T STAT methods.

District hospital cTnT (<0.1 [micro]
 g/L) (a) by
 CARDIAC T Rapid

March 29, 2001, at 1140 0.59
(sample 1); whole blood
(lithium heparinate)

March 30, 2001, at 1330 0.41
(sample 2); whole blood
(lithium heparinate)

Central laboratory cTnT (<0.1 [micro]
 g/L) (a) by
 T STAT

March 30, 2001, at 2030 <0.02
(plasma)

March 31, 2001, at 0800 <0.02
(plasma)

Plasma from sample 1 0.36 <0.02

Plasma from sample 2 0.49 <0.02

Patient whole blood 0.52
(lithium heparinate);
collected May 4, 2001,
at 1405 (sample 3)

Patient serum; collected <0.02
May 4, 2001, at 1405

Plasma from sample 3 0.39

200 [micro]L of plasma + 0.17
25 [micro]L of mouse
serum

200 [micro]L of plasma + <0.1 (0.05-0.09)
50 [micro]L of mouse
serum

200 [micro]L of plasma + Negative
75 [micro]L of mouse
serum

(a) Values in parentheses are upper reference limits.
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Title Annotation:Letters
Author:Kadri, Nadine; Douville, Pierre; Lachance, Pierre
Publication:Clinical Chemistry
Article Type:Letter to the editor
Date:Jun 1, 2002
Words:745
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