Interference by paraproteins is an infrequent finding in nine Roche c702 assays

Dear Editor,

Flowers et al ¹ recently published a case report demonstrating negative interference in the Roche enzymatic creatinine method caused by a moderate (12–15 g/L) IgM-kappa paraprotein. We too have encountered negative interference in the Roche enzymatic creatinine assay thought to be due to a similar sized, stable IgM-kappa paraprotein. The interference was first identified due to an unexpected low creatinine result in a 77-year-old male patient with Waldenstrom’s macroglobulinaemia. Inconsistent results were obtained on repeat analysis, even when the sample was diluted or pre-incubated at 37°C. The reaction curve showed an increasing absorbance after addition of reagent 1, followed by unstable reaction kinetics during the reaction monitoring window. The sample was referred to a neighbouring laboratory and analysed using an enzymatic creatinine method on a Beckman AU5800. Consistent creatinine results were obtained on replicate analyses, and reaction curves were normal. Subsequent samples from the same patient gave analytically plausible Roche enzymatic creatinine results. This led us to consider that we may be missing subtle interference from paraproteins in other patients and in other assays.

A review of the literature identified total bilirubin,^2–4 HDL cholesterol^3,4 and phosphate ⁵ as additional Roche assays known to be affected by interference from paraproteins. Other assays reported to be affected by interference include albumin (bromcresol green), ⁸ urate (uricase), ⁶ calcium (arsenazo-III), ⁷ iron (ferrozine) ⁹ and total protein (biuret). ¹⁰ Roche include a disclaimer for all of these assays stating that gammopathy may cause unreliable results. We identified 102 samples from different patients received in our laboratory for paraprotein monitoring. Type and concentration of paraprotein were documented before the samples were anonymised and analysed for creatinine and all the above tests on a Roche Cobas c702. The reaction curve from each test on each sample was reviewed by eye by either an experienced BMS or Clinical Scientist.

Paraprotein types were: IgA-kappa (n = 6); IgA-lambda (n = 6); heavy chain-only IgG (n = 1); IgG-kappa (n = 46); IgG-lambda (n = 17); IgM-kappa (n = 18); IgM-lambda (n = 8). Range of paraprotein concentrations: 5.3–71.5 g/L.

No abnormal reaction curves were identified for creatinine or any of the other analytes tested, except for total bilirubin in five patients. In these cases, interference was detected by the analyser software as a prozone error and samples were automatically repeated on dilution. Reaction kinetics were normal on the diluted samples. All five patients had a paraprotein >25 g/L (4 IgG-kappa, 1 IgG-lambda), although not all patients with a paraprotein >25 g/L generated the prozone error. In addition, the bilirubin reaction curves from 100 patients with no known paraprotein were reviewed. The prozone effect did not occur in any of these samples suggesting it was specifically caused by the paraprotein.

Our investigation of 102 samples with significant paraproteinemia has not detected any spurious results in the nine assays tested on the Roche c702. Reporting incorrect results due to paraprotein interference does not appear to be prevalent based on our data for the Roche 702 analyser. However, cases such as that reported by Flowers et al ¹ and ourselves highlight the need to investigate further when results do not fit with previous results or the clinical picture.