Effect of a variable magnetic field on clinical laboratory testing.

The performance of clinical analyz-ers is commonly assessed under a variety of environmental conditions to set acceptable limits (e.g., temperature and humidity) for operation. We recently faced an unusual environmental challenge within our automation laboratory, namely a variable magnetic field emanating from a Niobe௡ Magnetic Navigation System (Stereotaxis; http:// www.stereotaxis.com) being installed in the cardiac catheter laboratory immediately above our laboratory. The system contains 2 strong permanent magnets that generate magnetic fields of 0.08 – 0.1 T (800 –1000 G) in any direction. When the magnets are in the stowed position, the maximum magnetic field is 0.2 T (2000 G). Although more prominent for other medical devices, the permissible magnetic field strength is not an environmental variable usually considered by manufacturers of clinical analyzers (1, 2). We thus faced a multifaceted problem: On the day of the magnet delivery, we had to assess its effects on all of the analyzers within our automation laboratory, and we had to assess its effects on replacement analyzers due to be installed in the following weeks. Accordingly , we needed to assess the effects of a variable magnetic field on the accuracy and precision of the current and future clinical analyz-ers within our automation labora-in particular those that used magnetic particle– based reagents (Im-mulite 2500). We had replaced all cathode ray tube displays in the laboratory with liquid crystal display screens before magnet installation because the expected magnetic field was known to interfere with cathode ray tube operation. Our assessment study included repeated replicate analyses of QC materials (high, medium, and low concentrations of each analyte) before installation of the magnet, in the presence of the relatively low and constant magnetic field when the magnets were stowed, and in the much higher but transient magnetic field when the magnets were in motion. To eliminate any bias due to the vested interests of either the clinical laboratory or the catheter laboratory , we had an independent external consultant supervise the evaluation and analyze the data. Our goal was to determine whether a statistically significant change in results occurred after the magnet was installed. Given the number of experiments to be evaluated in a short period, we based our assessment on a 99% confidence level (rather than 95%). The highest magnetic field measured at any location in the laboratory when the magnets were stowed or moving was Ͻ1.26 G (Ͻ0.126 mT). This relatively weak field [occupational …