Confidence limits of arteriovenous fistula flow rate measured by the "on-line" thermodilution technique.

Background: A method is presented for estimating the confidence limits (CLs), or accuracy, of the arteriovenous fistula flow rate measured at haemodialysis by the "on-line" thermodilution technique.

Methods: This was by derivation of an expression to estimate what variance a set of repeated measures of flow would yield, using values pertaining to a single measure of flow. (Laws of variance were applied to the formula used to calculate flow, to account for its variables' values and measurement errors.) This enabled CLs of a single measure to be estimated.

Results: The variance estimated from a single measure was compared with that actually observed upon immediately taking a second measurement; differences in 189 pairs were not significantly different from zero (P=0.56). Applying the results demonstrated that measured flow values of 430-570 ml/min typically had associated 95% CLs that included 500 ml/min; therefore, true flow could not be said to be either side of 500 ml/min. The same was the case for 500-700 ml/min with regard to 600 ml/min. CLs widened considerably with the magnitude of flow rate, limiting the accurate measurement of higher flows and the detection of falls in flow.

Conclusion: A method to estimate CLs of flow rate measured by the thermodilution technique is presented and validated. Application demonstrates an accurate measurement of low flow, but limitations at higher flow and in detecting falls in flow. Appreciating the magnitude of such is critical to informed clinical decision making when using flow rate in an access surveillance programme.