Differences in prescribed Kt/V and delivered haemodialysis dose--why obesity makes a difference to survival for haemodialysis patients when using a 'one size fits all' Kt/V target.

BACKGROUND Morbid obesity is reported to be a survival factor for haemodialysis patients compared with those with a normal body mass index (BMI), yet morbid obesity (BMI >35) is a mortality risk factor for obese patients in the general population. Traditionally, haemodialysis dosing is prescribed to achieve a target Kt/V corrected for total body water (TBW). As obese patients typically have increased body fat, which contains less water than muscle, then obese patients may have lower levels of body water than their slimmer counterparts, and as such delivered Kt/V could be greater than that estimated using standard anthropomorphic equations, and so increased dialysis dose may help explain the increased survival reported for obese patients. METHODS We compared multi-frequency bioelectrical impedance analysis (MF-BIA) measurements of TBW in healthy haemodialysis outpatients, and compared TBW with that calculated from the Watson equation derived from anthropomorphic measurements. RESULTS Three hundred and sixty-three adult patients, mean age 58.1 ± 16.7 years, 60.9% male and 29.9% diabetic were studied. MF-BIA-measured body composition showed that as BMI increased from <20 to >35, the percentage skeletal muscle mass fell from 42.8 ± 4.9 to 29.2 ± 5.5% (P < 0.01) whereas the % fat mass increased from 24.5 ± 11.6 to 46.7 ± 6.8% (P < 0.01). As such, TBW measured by MF-BIA was significantly lower from that predicted by the Watson equation at higher BMIs (BMI > 35; 38.9 ± 6.8 versus 44.7 ± 6.9 L, P < 0.01 and BMI = 30-35; 37.2 ± 7.5 versus 41.9 ± 7.3 L, P < 0.01), and as such the delivered Kt/V using MF-BIA was much greater than that using the Watson-based urea volume of distribution for the obese patients (BMI > 35; spKt/V 1.63 ± 0.48 versus 1.41 ± 0.35 and BMI 30-35; 1.65 ± 0.3 versus 1.46 ± 0.26, both P < 0.01). CONCLUSIONS Prescribing dialysis or quantifying on-line clearance based on the anthropomorphically derived Watson equation leads to underestimation of the delivered dose to obese patients, due to changes in underlying body composition. As such, when using a 'one size fits all' target Kt/V, obese patients have an advantage over patients with normal BMI, in that they will receive a greater delivered dose of dialysis, and this may potentially explain the paradoxical survival advantage of the morbidly obese haemodialysis patient.