Abstract
Background: The i-STAT (Abbott Laboratories, Princeton, NJ) and the RapidPoint 500 (Siemens, Norwood, MA) point-of-care analyzers each generate oxygen saturation values at the bedside by applying an on-board algorithm to the oxygen tension (PaO2) figure generated by their internal miniaturized Clark electrodes. We sought to compare the accuracy of the digital readouts supplied by each analyzer, both to each other, and also to a polynomial expression first described in 1966, when applied to a homeostatic arterial blood gas (ABG) data set (pH = 7. 40; PaCO2 = 40 torr; [HCO3-] = 24.0 mEq/L).
Methods: We implemented the i-STAT's and the RapidPoint's algorithms for this data set for integer values of PaO2ranging between 10 and 100 torr. This succeeded in generating an oxyhemoglobin dissociation curve (OHDC) for each of the analyzers. We then generated an actual OHDC using a polynomial equation originally described by Kelman1. The OHDCs for the Kelman Equation, the i-STAT's algorithm, and the RapidPoint's algorithm were plotted on the same grid, using Numbers spreadsheet software (Apple, Cupertino, CA). This provided us with a visual comparison of the respective accuracy of the analyzers over a broad range, as compared to the Kelman Equation, which has long been considered to be the accepted standard.
Results: The accuracy of each of the analyzers was observed to be impressive, with the RapidPoint being slightly more accurate than the i-STAT. In the Figure displaying below, the actual OHDC, generated using the Kelman Equation, displays in red, while the RapidPoint-generated OHDC displays in blue, and the OHDC referable to the i-STAT displays in black.
Conclusions: In this mathematical modeling study of oxyhemoglobin saturation, the accuracy of the oxygen saturation readouts from two point-of-care analyzers were found to be within two percent of the actual value throughout the physiologic range, in the presence of homeostasis. Furthermore, the OHDCs generated by the point-of-care analyzers were strikingly similar to the corresponding curve generated by a methodology that has long been considered to be the accepted standard, lying well within one percent of the prevailing actual saturation at PaO2 values lying between 23 and 100 torr.
1. Kelman GR. Digital computer subroutine for the conversion of oxygen tension into saturation. J Appl Physiol 1966;21:1375-1376.
Footnotes
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