Calculators · Nephrology · Formula

Urea reduction ratio and single-pool Kt/V (Daugirdas II)

Also called URR, spKt/V, Kt/V, dialysis adequacy

Haemodialysis dose from pre- and post-dialysis BUN, session length and ultrafiltration.

mg/dL

Use the same unit for pre and post.

mg/dL
L
kg

Fill 2 more fields to see the result.

Formula and scoring

URR = (1 − post/pre) × 100. spKt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × UF/W, R = post/pre BUN, t in hours, UF in L, W in kg.

Exam and ward pearls

  • URR ignores urea removed by ultrafiltration, so Kt/V is the preferred adequacy measure.
  • The 0.008 × t term corrects for urea generated during the session; for 4–6 sessions a week the factor must be changed (Daugirdas 2012).

When not to rely on it

  • The 0.008 urea-generation factor is for thrice-weekly schedules; frequent or nocturnal schedules need a different factor.
  • Not for peritoneal dialysis.

Source

  1. Owen WF Jr, Lew NL, Liu Y, Lowrie EG, Lazarus JM. The urea reduction ratio and serum albumin concentration as predictors of mortality in patients undergoing hemodialysis. N Engl J Med. 1993;329(14):1001-6. doi:10.1056/NEJM199309303291404 · PMID 8366899
  2. Daugirdas JT. Second generation logarithmic estimates of single-pool variable volume Kt/V: an analysis of error. J Am Soc Nephrol. 1993;4(5):1205-13. doi:10.1681/ASN.V451205 · PMID 8305648
  3. Daugirdas JT, Leypoldt JK, Akonur A, Greene T, Depner TA. Improved equation for estimating single-pool Kt/V at higher dialysis frequencies. Nephrol Dial Transplant. 2013;28(8):2156-60. doi:10.1093/ndt/gfs115 · PMID 22561585

Formula and cut-offs checked against these sources on 8 Nov 2026. Educational tool for clinicians; it does not replace clinical judgement.

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