High myoglobin plasma samples risk being reported as falsely low due to antigen excess - follow up after a 2-year period of using a mitigating procedure.

Objectives: Antigen excess (AE) or high dose hook-effect can seriously affect the reported concentration of an analyte, which may impact clinical decisions. This study analyze to what extent the Roche Elecsys^® Myoglobin assay is affected by antigen excess and how this can affect the reported myoglobin concentration.

Methods: A dilution series experiment was conducted on a patient sample with a very high myoglobin concentration (>150,000 μg/L). Based on this, an AE mitigation procedure based on sample dilutions was developed and implemented. A retrospective analysis of reported patient results was performed to assess the potential frequency of samples that could be affected by AE.

Results: In the dilution experiment the Roche Elecsys^® Myoglobin assay was susceptive to AE, where samples with concentrations >115,000 μg/L may be reported as <3,000 μg/L. Dilution series from patient samples (n=20) subjected to the AE mitigation procedure showed the same pattern as the dilution experiment. The percentage of samples >115,000 µ/L increased significantly after the AE mitigation procedure had been implemented. After implementation of the AE mitigation procedure, the maximum myoglobin concentration observed was 780,000 μg/L.

Conclusions: If the Roche Elecsys^® Myoglobin assay is used according to the manufacturer, extremely high myoglobin samples risk being reported as only mildly elevated or near normal. This may affect clinical decisions. Thus, we suggest an AE mitigation procedure with automatic dilution of samples where myoglobin concentrations exceed 500 μg/L to mitigate this risk. Roche needs to take further actions to eliminate the consequences of the AE interference.