What is Kt/V in Hemodialysis?
Kt/V is a dimensionless measure of dialysis adequacy that quantifies the fractional urea clearance per dialysis session, normalized to the patient's total body water volume. 1
Core Components
The formula fundamentally represents three key variables 1:
- K = dialyzer urea clearance rate (mL/min)
- t = time on dialysis (minutes or hours)
- V = patient's total body water volume (liters)
The basic calculation uses the natural logarithm formula: Kt/V = ln(C0/C), where C0 is predialysis urea concentration and C is postdialysis urea concentration. 1
Practical Calculation Methods
Single-Pool Kt/V (spKt/V)
The simplified formula recommended by the American Journal of Kidney Diseases is 2, 1:
spKt/V = -ln(R - 0.008 × t) + (4 - 3.5 × R) + (UF/W)
Where:
- R = ratio of postdialysis BUN to predialysis BUN
- t = dialysis time in hours
- UF = ultrafiltration volume in liters
- W = postdialysis weight in kilograms
This equation accounts for the three major determinants: the decrease in urea concentration during dialysis, contraction of body water volume during dialysis, and generation of urea during treatment. 2
Equilibrated Kt/V (eKt/V)
For patients requiring more precise measurement, equilibrated Kt/V accounts for urea disequilibrium using 1:
eKt/V = spKt/V - (0.6)(K/V) - 0.03
However, the American Journal of Kidney Diseases guidelines elected to stay with spKt/V as the established standard due to the additional effort required for eKt/V calculation. 2
Clinical Target
The minimum target spKt/V is 1.2 per session for patients on thrice-weekly hemodialysis, which translates to approximately 3.6 per week. 1
Critical Considerations
Ultrafiltration Component
The ultrafiltration volume contributes significantly to total urea clearance and must be included in dose calculations, particularly when fluid removal volumes are large. 1, 3 This component can account for a substantial portion of the overall Kt/V value. 3
Gender and Body Size Bias
Using V as the denominator creates bias in women and smaller patients, potentially leading to underdialysis despite achieving target Kt/V values. 1, 4 A target Kt/V of 1.2 corresponds to a range of equivalent doses that are significantly higher in males than females and in larger than smaller males. 4
Residual Kidney Function
Native kidney urea clearance should be added to the dialysis dose calculation when present, as residual kidney function contributes more to improved outcomes than equivalent dialyzer clearance. 1 However, omission will protect the patient from underdialysis when residual kidney function is lost. 2
Treatment Frequency Adjustments
Kt/V values cannot be directly compared across different treatment frequencies without adjustment. 1 For patients receiving four treatments per week instead of three, the minimum dose per dialysis required is 0.9 per dialysis (rather than 1.2) to achieve the same weekly clearance. 2
Alternative Measurement Methods
Online Clearance Monitoring
Conductivity (ionic) clearance can be measured across the dialyzer membrane without blood sampling, providing real-time Kt/V prediction before treatment completion. 2 However, this method requires accurate determination of V and multiple measurements throughout treatment to avoid errors from changes in clearance during dialysis. 2
Urea Reduction Ratio (URR)
URR = (C0 - C) / C0 correlates well with spKt/V in population studies but has significant variability in individual patients because it fails to include extracellular volume contraction and urea generation during dialysis. 2 Despite this limitation, when outcomes including death are correlated with either URR or Kt/V, no difference in degree of correlation is detectable. 2
Quality Assurance Application
Comparison of prescribed versus delivered doses serves as a quality assurance tool. 2 When significantly different (15% difference), troubleshooting should detect problems such as access recirculation, faulty blood pump, inadequate dialyzer performance, or errors in blood sampling. 2