Use of vascular access blood flow to evaluate vascular access

Use of vascular access blood flow to evaluate vascular access

CORRESPONDENCE Letters to the Editor are considered for publication (subject to editing and abridgement without notice) provided they are submitted in...

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CORRESPONDENCE Letters to the Editor are considered for publication (subject to editing and abridgement without notice) provided they are submitted in one of two ways. If typewritten, they must be submitted in duplicate, double-spaced, and not more than 40 typewritten lines of manuscript text (excluding references). You may also e-mail your letters to [email protected]. The same length limits apply. Letters should not duplicate similar material being submitted or published elsewhere. Letters to a recent Journal article must be received within 6 months of the article’s publication. Receipt of letters is not acknowledged and there is no guarantee that your letter will be published. Submitting the letter constitutes your permission for its publication in any current or subsequent issue or edition of the Journal, in any form or media, now known or hereafter developed.

probabilities from Table 1. We have included criteria of Qa ⬍800 mL/min or ⌬Qa ⱖ10% so as to increase the sensitivity of Qa in predicting thrombosis. These results show that a high sensitivity requires a high FPR (which would result in many unnecessary procedures). Comparison of post-test probabilities shows that Qa failed to accurately discriminate between thrombosis and nonthrombosis. To summarize, either/or use of single Qa with ⌬Qa was an inaccurate predictor of thrombosis in our patients. Further studies are needed to provide a more complete picture of the role of Qa surveillance in reducing graft thrombosis. Until such studies are available, we believe that it is premature to recommend Qa surveillance with intervention of all patients with grafts. William D. Paulson, MD Sunanda J. Ram, PhD Division of Nephrology Louisiana State University Health Sciences Center Shreveport, Louisiana Jack Work, MD Emory University School of Medicine Atlanta, Georgia

USE OF VASCULAR ACCESS BLOOD FLOW TO EVALUATE VASCULAR ACCESS To the Editor: We have recently engaged in debates with Hakim and Ikizler1,2 and Krivitski and Gantela3,4 on the accuracy of hemodialysis graft blood flow (Qa) surveillance in predicting graft thrombosis. This debate prompted us to review the data from our studies.5,6 We had reported that an either/or prediction of thrombosis by Qa ⬍600 ml/min or a decrease in Qa (⌬Qa) ⱖ20% had a sensitivity of 77% and a false-positive rate (FPR) of 23% in predicting thrombosis (p 1093).6 We have found that this sensitivity was actually the specificity (1 ⫺ FPR) and that the true sensitivity was 59%. Examination of the single Qa data in Fig 2 of Paulson et al5 and ⌬Qa data in Fig 5 of Paulson et al6 shows that such a high sensitivity with low FPR was not possible. We regret this error, because it has led Krivitski and Gantela3 to assume an erroneously high accuracy for Qa in predicting thrombosis. Table 1 shows the data from 83 grafts that we used to compute the sensitivity and FPR.6 Note that Qa failed to warn of 18 of 44 thromboses. Table 2 computes relevant Table 1. Raw Data Used to Determine Accuracy of Qa and sQa (either/or) in Predicting Thrombosis

Graft Outcome

Not thrombosed Thrombosed

Negative Test (Qa ⱖ 600 mL/min and ⌬Qa ⬍ 20%)

Positive Test (Qa ⬍ 600 mL/min or ⌬Qa ⱖ 20%)

30 18

9 26

Table 2.

Criteria for Positive Test Result

Qa ⬍ 600 mL/min or ⌬Qa ⱖ 20% Qa ⬍ 800 mL/min or ⌬Qa ⱖ 10%

916

REFERENCES 1. Hakim R, Ikizler TA: Use of vascular access blood flow to evaluate vascular access. Am J Kidney Dis 37:451, 2001 (letter) 2. Paulson W, Ram SJ, Work J: Use of vascular access blood flow to evaluate vascular access. Am J Kidney Dis 37:451-452, 2001 (letter) 3. Krivitski NM, Gantela S: Access flow measurement as a predictor of hemodialysis graft thrombosis: Making clinical decisions. Semin Dial 14:181-185, 2001 4. Paulson WD: Blood flow surveillance of hemodialysis grafts and the dysfunction hypothesis. Semin Dial 14:175-180, 2001 5. Paulson WD, Ram SJ, Birk CG, Work J: Does blood flow accurately predict thrombosis or failure of hemodialysis synthetic grafts? A meta-analysis. Am J Kidney Dis 34:478-485, 1999 6. Paulson WD, Ram SJ, Birk CG, Zapczynski M, Martin SR, Work J: Accuracy of decrease in blood blow in predicting hemodialysis graft thrombosis. Am J Kidney Dis 35: 1089-1095, 2000 © 2001 by the National Kidney Foundation, Inc. doi:10.1053/ajkd.2001.28795

Accuracy of Qa and sQa (either/or) in Predicting Thrombosis

Sensitivity

False-Positive Rate

Thrombosis Prevalence (pretest probability)

Posttest Probability of Thrombosis if Positive Test Result

Posttest Probability of Thrombosis if Negative Test Result

59% 73%

23% 51%

53% 53%

74% 62%

38% 39%

American Journal of Kidney Diseases, Vol 38, No 4 (October), 2001: p 916