Predicting Dialysis Fistula Maturation Using the "Rule of 6'S"-Not All are Created Equal

The rule of 6's (Ro6; flow volume >600 mL/min; vein diameter >6 mm; vein depth <6 mm) is a widely used informal guide to determine when an arteriovenous fistula (AVF) will support dialysis. We tested the utility of the Ro6 in clinical practice. We retrospectively reviewed AVFs created at a single center from 2016 to 2019 in patients who had received dialysis in the same network. For each AVF, we reviewed the clinical records and postoperative ultrasound studies (including flow measurement) for Ro6 criteria. Maturation was defined as use of the AVF with two needles for 75% of dialysis sessions during a continuous 4-week period with mean flow of 300 mL/min or Kt/V of 1.2. The predictors of maturation were assessed using logistic regression and receiver operating characteristics curves after stratifying AVFs by type. P < .05 was considered significant. A total of 202 AVFs of three different types met the inclusion criteria (radiocephalic, n = 49; brachiocephalic, n = 87; brachiobasilic, n = 66). Maturation occurred in 150 AVFs (74%) (primary, n = 101 [50%]; assisted, n = 49 [24%]), and 52 (26%) failed to mature. Maturation did not vary by AVF type, sex, or diabetes status. However, a higher body mass index (BMI) was associated with failure to mature (P = .004). For all types, one or more Ro6 criteria were satisfied in significantly more mature AVFs than in those that failed to mature (55% vs 36%; P = .009). Only 16 mature AVFs (11%) met all three Ro6 criteria; however, all mature AVF met more than one criterion. On multivariable analysis (Table I), each Ro6 criterion was independently associated with maturation. If all three were met, the AVF was 10 times more likely to be mature compared with an AVF meeting no criteria. The BMI correlated strongly with vein depth (P < .001); however, both characteristics independently predicted for maturation. The chance of maturation was highest if the flow and depth criteria were met (positive predictive value, 93), marginally better than if all three criteria were met (positive predictive value, 92; Table II). The receiver operating characteristic area under curve for meeting the flow volume and vein depth criteria together were slightly greater than if all three Ro6 criteria were met (also true for each AVF type; data not shown). The Ro6 predicts for AVF maturation but only a small minority of mature AVFs will meet all three criteria. Flow volume and vein depth together predicted for maturation equally as well as meeting all three criteria. Meeting the vein diameter criterion seems less important. A higher BMI was associated with AVF failure, independent of vein depth.Table IMultivariable logistic regression of independent predictors of functional AVF maturationVariableOR95% CIP valueAny flow volume >600 mL/min6.773.09-14.866 mm4.051.53-10.76.005All three criteria9.804.40-21.82600 mL/min807390560.765Any depth <6 mm874482540.655Any diameter >6 mm912778500.588All three criteria688392470.754Flow + diameter only747389490.735Flow + depth only748393520.784Diameter + depth only776787510.732AUC, Area under the curve; AVF, arteriovenous fistula; ROC, receiver operating characteristic; NPV, negative predictive value; PPV, positive predictive value. Open table in a new tab