Rules of 6 criteria predict dialysis fistula maturation but not all rules are equal

The Rules of 6 (flow volume >600 mL/min, vein diameter >6 mm, vein depth <6 mm) are widely used to determine when an arteriovenous fistula (AVF) will support dialysis. Thus, we tested the utility of the Rules of 6 in clinical practice.

We retrospectively reviewed AVFs created at a single center from 2016 to 2019 for patients who had undergone dialysis within the same healthcare system. Clinical records and postoperative ultrasound studies were reviewed for the Rules of 6 criteria. Maturation was defined as use of the AVF with two needles for 75% of the dialysis sessions for a continuous 4-week period, with a mean flow of 300 mL/min or urea clearance (Kt/V) of 1.2. Predictors of maturation were assessed using logistic regression and receiver operating characteristic (ROC) curves.

Five surgeons performed 202 AVFs of three types during 2016 to 2019 (radial-cephalic, n = 49; brachial-cephalic, n = 87; brachial-basilic, n = 66). Maturation occurred in 150 AVFs (74%; primary, n = 101 [50%]; assisted, n = 49 [24%]), while 52 (26%) failed to mature. Maturation did not vary by AVF type or patient sex or diabetes status. A higher body mass index was associated with failure to mature (P = .004). Only 16 mature AVFs (11%) met all three Rules of 6 using mean values for flow, diameter, and depth. However, 101 (67%) met all three Rules using the extreme, maximum or minimum, values. On multivariate analysis, each Rule of 6 was independently associated with maturation. If all three Rules were met, the AVF was nearly 10-fold more likely to have matured compared with an AVF satisfying no Rule. The body mass index correlated strongly with the vein depth (P < .001); however, both characteristics independently predicted maturation. The chance of maturation was highest if flow and depth Rules were met (positive predictive value [PPV], 93%); if all three rules were met, the PPV was 92%. The ROC area under curve (AUC) values for meeting flow volume and vein depth Rules together were higher than if all three Rules had been satisfied (0.784 vs 0.754). The PPV for diameter alone (78%) was the lowest of all PPVs for the three Rules and the ROC-AUC was only 0.588. If all three Rules together were not satisfied using extreme values, the negative predictive value was only 47%.

The Rules of 6 predict AVF maturation, especially when using extreme, maximum or minimum, values to satisfy each Rule. Flow volume and vein depth together predict maturation equally as well as meeting all three Rules. Vein diameter seems less important. The Rules of 6 might be too stringent if used exclusively to predict for functional AVF maturation.