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Stable chronic kidney disease, hypertension and obesity

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A BNF case study involving a man with management of stable chronic kidney disease, hypertension and obesity.

Case study

A 61 year old Caucasian man is reviewed for the management of stable chronic kidney disease, hypertension, and obesity. He has never smoked, and does not have diabetes or significant proteinuria.

  • Blood pressure = 145/95mmHg (6 months ago it was 165/102mmHg)
  • Serum potassium = 4.4mmol/litre
  • Serum creatinine  = 260micromol/litre
  • eGFR = 23mL/minute/1.73m2 
  • Fasting total cholesterol = 3.4mmol/litre
  • Liver function and other U + Es are normal
  • Height = 175cm (5 feet 9 inches), weight = 98kg

His medications include: lisinopril 20mg daily, bendroflumethiazide 2.5mg daily, simvastatin 40mg daily

What is this patient’s body mass index?

According to the BNF online calculator, his body mass index = 32kg/m2.

Which measures of renal function should be used to make drug dose adjustments in this patient?

Prescribing in Renal Impairment in the BNF states that for most drugs, information on dosage adjustments for renal impairment in the BNF is expressed in terms of eGFR. However, in this patient with a body mass index greater than 30kg/m2, either the absolute glomerular filtration rate or the creatinine clearance (calculated from the Cockcroft and Gault formula) should be used in place of the eGFR to adjust drug doses. For potentially toxic drugs with a small safety margin, the creatinine clearance should be used to adjust drug doses in conjunction with plasma-drug concentration and clinical response.

What is this patient’s body surface area?

According to the BNF online calculator, his body surface area = 2.13m2.

What is this patient’s absolute glomerular filtration rate?

The eGFR reported by the laboratory can be converted to the absolute glomerular filtration rate using the following equation:

GFRAbsolute =  eGFR x (individual’s body surface area/1.73)

GFRAbsolute =  23 X (2.13/1.73) = 28mL/minute 

Why should you use the ideal body weight to calculate this patient’s creatinine clearance?

He is overweight for his height and his actual body-weight will overestimate his creatinine clearance. His ideal body weight can be calculated as follows:

Ideal body weight in men (kg) =  50 + 2.3 x (every inch above 5 feet)

Ideal body weight in men (kg) =  50 + (2.3 x 9) = 70.7kg

What is this patient’s creatinine clearance?

According to the BNF online calculator for creatinine clearance (using the Cockcroft and Gault formula), his creatinine clearance is 26mL/minute.

In this patient, the GFRAbsolute is similar to the calculated creatinine clearance, and either value can be used to adjust drug doses.

How will you optimise this patient’s antihypertensive treatment?

The prescribing notes on Hypertension in section 2.5, BNF 58, recommend that the optimal blood pressure target in patients with renal impairment is a systolic blood pressure < 130mmHg and a diastolic blood pressure < 80mmHg, or lower if proteinuria exceeds 1g in 24 hours.

As bendroflumethiazide and lisinopril do not have a narrow therapeutic index, either the GFRAbsolute or creatinine clearance can be used to adjust their doses in this patient. According to the prescribing notes on Thiazides and Related Diuretics in section 2.2.1 of the BNF, bendroflumethiazide should be avoided if the eGFR is less than 30mL/minute/1.73 m2 because it will be ineffective. As this patient’s GFRAbsolute and creatinine clearance is less than 30mL/minute, the bendroflumethiazide should be stopped.

According to the monograph for lisinopril, the dose of lisinopril can be titrated to a maximum of 40mg daily if the eGFR is 10-30mL/minute/1.73 m2. As this patient’s GFRAbsolute and creatinine clearance is between 10-30mL/minute, the dose of lisinopril can be titrated to a maximum of 40mg daily.  However, patients over 55 years of age may respond less well to treatment with an ACE inhibitor. The prescribing notes on ACE inhibitors in section 2.5.5.1, BNF 58, advise that renal function and electrolytes should be monitored during treatment with lisinopril; hyperkaleamia and other side-effects are more common in those with impaired renal function and may limit the dose of lisinopril that can be used. Although ACE inhibitors occasionally exacerbate renal impairment, particularly in patients with renovascular disease, this patient has no features of renovascular disease.

If lisinopril alone is inadequate at controlling blood pressure, then a dihydropyridine calcium channel blocker or a loop diuretic can be added (section 2.5, BNF 58).

Should simvastatin be continued at the current dose?

As simvastatin does not have a narrow therapeutic index, either the GFRAbsolute or creatinine clearance can be used to adjust its doses in this patient. According to the BNF monograph for simvastatin, doses above 10mg daily should be used with caution if the eGFR is less than 30mL/minute/1.73 m2. Although this patient’s GFRAbsolute and creatinine clearance is less than 30mL/minute, he is not experiencing any side-effects usually associated with statins. Therefore, simvastatin can be continued at 40mg daily with appropriate monitoring. Alternatively, he could be switched to either atorvastatin or fluvastatin, which do not require dose adjustments during renal impairment.    

 

Which anti-obesity drug can be prescribed?

The prescribing notes on obesity in section 4.5, BNF 58, advise that an anti-obesity drug should be considered in those with a body mass index of 30kg/m2 or more in whom at least 3 months of managed care involving supervised diet, exercise and behaviour modification fails to achieve a realistic reduction in weight. Sibutramine is contra-indicated in uncontrolled hypertension or if the eGFR is less than 30mL/minute/1.73 m2. Sibutramine is inappropriate for this patient because he has uncontrolled hypertension and his GFRAbsolute and creatinine clearance is less than 30mL/minute. Orlistat can be used in conjunction with other lifestyle measures to manage obesity and requires no dose adjustment for renal impairment.

Case study

18 months later, the patient’s body-weight = 80kg, body mass index = 26kg/m2, blood pressure = 125/75mmHg, serum potassium = 4.8mmol/litre, serum creatinine = 215micromol/litre, eGFR = 29mL/minute/1.73m2.

His medications include:

  • lisinopril 40mg daily
  • amlodipine 10mg daily
  • aspirin 75mg daily
  • simvastatin 40mg daily

Which measures of renal function should be used to make drug dose adjustments in this patient?

According to Prescribing in Renal Impairment, as this patient’s body mass index is greater than 18.5kg/m2  but less than 30kg/m2, the laboratory reported eGFR can now be used to make dosage adjustments for most drugs. However, the creatinine clearance (calculated from the Cockcroft and Gault formula) should continue to be used for potentially toxic drugs with small safety margins.  

The patient develops acute pyelonephritis. What dose of oral ciprofloxacin should be prescribed?

As ciprofloxacin does not have a narrow therapeutic index, the eGFR can be used to make drug dose adjustments in this patient. According to the BNF monograph for ciprofloxacin, a dose of 250-500mg daily can be used if the eGFR is less than 30mL/minute/1.73m2.

Based on a case study from the BNF/BNFC e-newsletter, September 2009

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