Compute fractional urea clearance adjusted for ultrafiltration volume and fluid contraction using the clinically validated second-generation Daugirdas equation.
The Single-Pool Index (spKt/V) is a core clinical metric used in nephrology to assess the structural adequacy of a hemodialysis prescription. While simpler metrics like the Urea Reduction Ratio (URR) look only at the basic percentage drop in blood urea nitrogen, spKt/V provides a more comprehensive evaluation. It models the human body as a single, uniform fluid compartment (pool) and accounts for two critical factors: the urea generated during the session and the fluid contraction caused by ultrafiltration (weight removal). Utilizing the globally accepted second-generation Daugirdas formula, this calculator helps clinical teams verify that a patient's dialysis setup provides sufficient small-solute clearance to meet established international safety standards.
Formula:
Understanding the Mathematical Architecture of spKt/V
The single-pool Kt/V metric measures hemodialysis treatment effectiveness by combining three core elements into a single index: dialyzer clearance capacity (K), treatment time (t), and the patient's total urea distribution volume (V).
The Daugirdas Second-Generation Equation
To accurately track clearance dynamics without requiring complex, constant urine collection testing, clinical models use the Daugirdas Second-Generation Formula. This equation isolates fluid adjustments using a log-linear approach:
spKt/V = -ln( BUN Ratio - (0.008 × Duration) ) + [ (4 - (3.5 × BUN Ratio)) × (UF Volume / Target Weight) ]
Where individual component terms are defined as follows:
- BUN Ratio: The post-dialysis concentration divided by the pre-dialysis concentration (Post-BUN / Pre-BUN).
- Duration: Total unbroken dialysis machine runtime calculated explicitly in hours.
- 0.008 × Duration: An empirical correction term that adjusts for ongoing urea generation by the patient's liver during the treatment session.
- UF Volume: Total fluid weight removed from the body during the session (measured in Liters or kilograms).
- Target Weight: The final actual weight of the patient at the conclusion of the session (measured in kilograms).
- UF Volume / Target Weight: The fluid contraction factor. This accounts for the rise in post-dialysis blood concentration that occurs as excess water is cleared from the vascular space.
Clinical Interpretation Requirements
- The Minimum Target Baseline (1.2): International consensus standards (including KDOQI) require a minimum delivered spKt/V of 1.2 per session for patients on a standard thrice-weekly treatment schedule. This baseline ensures adequate waste clearance to support long-term cardiovascular health and general physical wellness.
- The Prescribed Target (1.4): Because individual sessions can experience minor real-world variations—such as access line recirculation, needle movement, or brief equipment interruptions—medical teams typically set the initial prescription target at 1.4. This built-in buffer helps guarantee that everyday real-world treatments reliably hit or exceed the absolute 1.2 safety minimum.