I have pasted in (vide infra) our draft proposal for adopting a GFR prediction, based on Cockroft & Gault. The Kidney Disease Outcome Quality Initiative (K/DOQI) evidence-based clinical practice guidelines state that "..GFR should be estimated from prediction equations..."and that "...using timed urine colections does not improve the estimate of GFR....". Other guidelines reviews (Clin Biochem Rev; vol 24, Aug 2003, 95-7) state that "the MDRD formula has not yet been subject to sufficient scrutiny to recommend its routine use."
CHRISTCHURCH HOSPITAL (NZ) PROPOSAL:
Proposal to offer "Calculated Glomerular Filtration Rate" using measured plasma creatinine and the Cockroft and Gault formula.
Creatinine clearance is can be estimated from the serum creatinine level, using the Cockroft and Gault formula (Nephron 1976 16:31-41). A modified version of this formula (see below) uses a nominal figure for the patient's ideal body weight based on their height, which has been shown to correlate more closely with measured 24 hr creatinine clearance (Intensive Care Med 1993 19:39-43). The same formula is currently in use at Christchurch Hospital ("Management Guidelines for Common Medical Conditions") for guidance of drug dosing. The proposal is to offer a calculation through Canterbury Health Laboratories.
CrCl = (140-age) x IBW x F / (Serum Cr x 48816)
· F = 1.0 for men or 0.85 for women.
· Men : IBW = 50 kg + 0.9 kg for every cm over 150 cm height
· Women: IBW = 45 kg + 0.9 kg for every cm over 150 cm height
In practice, taking the patient's self reported height to the nearest centimetre will be accurate enough for clinical purposes and it is not necessary to get the patient up to get an exact measurement during the current admission. Alternatively, if it is not possible to use height, then actual body weight can be used as an alternative.
Practical issues for requesting "calculated GFR".
Clinicians should write "calculated GFR" in the "other tests" section of the Canterbury Health Laboratories request form.
Either height (metres and centimetres) or weight (kg) should be given. Self reported height to the nearest centimetre is considered to be sufficiently accurate.
If both height and weight are given, then precedence will be given to height and GFR will be calculated based on ideal body weight.
If weight alone is given, then GFR will be calculated according to actual body weight.
Comments to be issued with reports:
For all reports:
Report : "calculated GFR based on ideal body weight, derived from height" OR
Report: "calculated GFR based on actual body weight"
For all reports based on ideal body weight:
"Under steady state conditions, calculated GFR is accurate enough to guide drug dosing, although may overestimate GFR in subjects with low Body Mass Index and underestimate GFR in the obese. Caution should also be exercised in interpretation in amputees and other complex cases."
For all reports based on actual body weight:
"Under steady state conditions, calculated GFR is accurate enough to guide drug dosing, although caution should be exercised in interpretation in extremes of body weight, amputees and other complex cases."
If no weight or height given:
"Height or weight not given to enable calculation of GFR".
If plasma creatinine < 0.06 mmol:
"Serum creatinine was too low for accurate estimation of GFR by the Cockroft-Gault method."
For Children (age <16 years) - no calculation will be done
"The Cockroft-Gault equation has not been fully validated in younger subjects".
Proposal under review, with strong impetus from nephrologists to support a move away from 24h urine collections. We expect this to be adopted.
Chris Florkowski
Christchurch, NZ
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