Paraprotein interference with turbidimetric gentamicin assay

Abstract

IntroductionGentamicin due to its low level of resistance and rapid bactericidal activity is commonly used to treat gram-negative bacteria. However, due to its toxic effects it needs to be monitored. To date, no interference has been reported with gentamicin assays.Materials and methodsA patient with leg cellulitis and sepsis received a single dose of gentamicin and a sample was sent for gentamicin analysis. The sample showed high blank absorbance readings on Beckman DxC800 and DC800 analysers with various dilutions. A second sample was received and analysed on a Roche Cobas system to obtain a result. A third sample was received 107 hours later with the same results and this sample was then analysed neat and post ethanol precipitation on all the turbidimetric assays available on the DxC800 analyser.ResultsThe high blank absorbance was observed upon addition of the reactive reagents due to protein precipitation. Although not obvious from the patient protein results, it was shown the presence of high IgM paraprotein, 18.9 g/L (reference range 0.4-2.3 g/L) was the cause of precipitation, giving high blank readings. Of all the other turbidimetric assays, only vancomicin and valproate showed similar high blank absorbance readings. To be able to provide more rapid results it was shown ethanol could be used as a precipitant of proteins in both calibrators and patient samples with acceptable recovery.ConclusionIgM paraprotein was identified as the cause of interference with the gentamicin, vancomicin and valproate assays. Protein interference in these assays can be overcome by precipitation with ethanol.

A Spuriously High Phenytoin Level and an Abnormal Lipemic Index: When Aberrant Laboratory Results Lead to a Medical Diagnosis

“…High Phenytoin Levels and Abnormal Lipemic Index others (5-10). Paraprotein interference with vancomycin, gentamycin, and valproate analyses has also been described, with falsely decreased drug levels or unreportable results flagged by the instrument (11)(12)(13). Although a spuriously low serum phenytoin level had been previously reported on the Siemens Dimension ® RxL and Vista systems (14,15), to our knowledge, this is the first case of a paraprotein causing an erroneously high phenytoin concentration.…”

Falsely Elevated Vancomycin Concentrations in a Patient Not Receiving Vancomycin

“…Polyethylene glycol (PEG) precipitation is a common, convenient, and nonspecific method used to detect and/or remove assay interference due to antibodies and other high molecular weight proteins in serum . A high vancomycin control of 38 mg/L was prepared.…”

“…Polyethylene glycol (PEG) precipitation is a common, convenient, and nonspecific method used to detect and/or remove assay interference due to antibodies and other high molecular weight proteins in serum. 2,4,5 A high vancomycin control of 38 mg/L was prepared. The sample was divided and diluted in a 1:2 ratio with 25% PEG6000 solution (Fluka#812160, PEG6000, Sigma Aldrich, St. Louis, MO) or phosphate-buffered saline (PBS), incubated, centrifuged (10 min at 13,200 g), and the concentrations of the supernatants compared.…”

“…When LC‐MS/MS is not available, alternative immunoassay platforms may be used to corroborate findings and assist in understanding the potential mechanism of interference with the original assay. In addition, protein precipitation, identification of other sources of interference (such as medications), and recognition of underlying pathologic abnormalities are important considerations …”

“…In addition, protein precipitation, identification of other sources of interference (such as medications), and recognition of underlying pathologic abnormalities are important considerations. 2,4,7 Falsely low vancomycin concentrations, secondary to immunoglobulin (Ig)-mediated interference, have been reported with PETINIA platforms including those manufactured by Beckman-Coulter (DxC 800, Synchron) [8][9][10] and Siemens (VISTA 1500). 5 In five publications involving seven cases of falsely low serum vancomycin concentrations, the reported results were considered suspect because all of the patients were receiving vancomycin before the time of serum sampling.…”