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Accuracy & Precision

Accuracy & Precision

Many publications cite device performance in terms of accuracy. Accuracy in this context is simply the ability to measure the actual Stroke Volume (SV) in millilitres. The pulmonary artery catheter (PAC) is regarded as the ‘gold standard’ in this respect but is known to be no better than ±20%.

For clinical use this is regarded as acceptable and within the clinical norm of ±30%. The basic measurement of Stroke Distance (SD) by ODM has an accuracy on a single waveform of ±3%. The conversion to SV through the nomogram results in a greater error, similar to that of a PAC.

However when using a device to guide IOFM in a 10% SVO protocol it is its precision that matters most of all, that is the ability to detect change in sequential measurements, in this case the effect of a fluid challenge on SD or SV. Precision is the ability to measure the same result repeatedly with minimal error.

Prof. Singer established that the error of repeatability of measuring SD for the ODM was 3.8% [1]. For an individual patient the diameter of the aorta will be a constant thus the SV precision will be equal to the known error for SD. [SV = SD x Aortic Root Diameter (from patient nomogram)].

This precision/repeatability error can then be used to determine the least significant change in SD/SV required to ensure confidence in measuring a real haemodynamic change and not just measurement error. With its calculated error of 3.8%, the user can be 99% confident that a measured change in SD/SV of >10% is real (this is based on 99% of normally distributed data points falling within 2.5 standard deviations of the mean).

The precision of a technology dictates its ability to guide fluid management. The 10% SV change algorithm  used to optimise SV is specific to the ODM, and is evidence-based. Other technologies that are less precise may not be as effective in guiding fluid management based on this algorithm.

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1Singer, M., J. Clarke, and E.D. Bennett, Continuous hemodynamic monitoring by esophageal Doppler. Crit Care Med, 1989. 17(5): p. 447-52