Prescribing guidance 'not being followed' in older people with reduced kidney function

Kidney function was found to be too low for recommended prescribing in 4–40% of people aged ≥65 years and 24–80% of people aged ≥85 years, despite over 90% of patients having recent kidney function test results recorded.

The study, conducted by researchers from the universities of Leeds and Bradford, included 70,900 older patients from 80 general practices.

It also showed that using eGFR overestimated kidney function for 3–28% of people aged ≥65 years, and for 13–58% of those aged ≥85 years.

Example drugs

Data were collected for age, sex, actual weight, serum creatinine and estimated glomerular filtration rate (eGFR). Kidney function as creatinine clearance (Cockcroft-Gault) was calculated using actual body weight and estimated ideal body weight.

The researchers used 8 example drugs to assess the extent to which prescribing was adjusted appropriately in people with reduced kidney function. The drugs were chosen based on findings of a previous case-note review of five practices in the PCT, the strength of recommendations in the prescribing literature, the impact on patients and expert independent advice from specialists.

Cockcroft-Gault or eGFR

The study’s findings underscore the need for prescribers to use the Cockcroft-Gault formula when adjusting drug dosage in older people with reduced kidney function, say the researchers.

The Cockcroft-Gault formula is thought to underestimate kidney function by 10% in older people whereas the eGFR is thought to overestimate it as age increases by as much as 69%. Using eGFR for prescribing decisions has been shown to increase the likelihood of adverse drug reactions, hospitalisation, or ineffectiveness.

Prescribing references consequently recommend the Cockcroft-Gault formula for estimating renal function or calculating drug doses in older people with reduced kidney function.

Drugs to review

The NICE acute kidney injury guideline alerts GPs to review medicines after an acute kidney injury episode. NICE guidance on chronic kidney disease also recommends the 'review of medicines'.

Drugs that pose a direct risk to the kidneys include NSAIDs, ACE inhibitors and angiotensin II receptor blockers (ARBs).

Drugs that are more likely to cause adverse reactions when blood levels are increased because of slower elimination include metformin, gabapentin and pregabalin.

Reduced kidney function can also reduce the effectiveness of some drugs such as thiazides and nitrofurantoin.