A study of discrepancy between laboratory-calculated and weight-based glomerular filtration rate (GFR) as a marker of renal function in a palliative care population.

58 Background: Renal function is important in a palliative care population and can impact on frequently-used medications including chemotherapeutic agents, opioids, non-steroidal anti-inflammatory drugs and neuropathic agents. In local practice, estimated glomerular filtration rate (eGFR), calculated on a presumed weight of 70kg, is displayed in laboratory results. However, the weight of palliative patients varies considerably, for example, due to cancer cachexia or malabsorption and a considerable population weigh less than 70kg. Our aim was to assess whether there is a discrepancy between glomerular filtration rate when laboratory-estimated versus weight-based and to explore the potential clinical relevance.

METHODS: Data was retrospectively collated for 50 patients admitted consecutively to a specialist palliative care unit over a 6 month period. Laboratory-derived eGFR was compared to GFR using the Cockroft-Gault equation (CreatClear = Sex*(140-Age)/(SerumCreat))*(Weight/72).

RESULTS: 25 (50%) patients were admitted for end of life care, 19 (38%) for symptom control, 6 (12%) for respite. 36 patients (72%) had weight documented. 45 (90%) patients had bloods taken. 41 patients had an eGFR documented, 7 of whom had an eGFR of less than 60ml/min. 25 patients had a weight-based GFR calculated of whom 10 (40%) had a GFR less than 60ml/min A significant discrepancy was seen between estimated and calculated GFR in 7 of these (accounting for 28%). This was most notable at less than 65kg. 2 patients with a weight of 55kg had a greater than 40ml/min difference in creatinine clearance. 50% of patients with a GFR of less than 60ml/min were on at least one nephrotoxic drug, most frequently pregabalin or ibuprofen. The most frequently-used opioid was oxycodone. No patient had opioids or medications discontinued on the basis of renal function.

CONCLUSIONS: Results show a significant discrepancy between estimated and calculated GFR in a palliative care population which is most pronounced at lower weights. We would recommend, where appropriate, calculating a weight-based GFR on patients, with a review of opioid and nephrotoxic medications when reduced.