A Diabetic Man with Swollen Ankles and a High Potassium
A 52-year-old man with diabetes for 18 years attends a clinic with ankle swelling, frothy urine and nocturia. Blood results: creatinine 3.4 mg/dL, urea 78 mg/dL, potassium 5.8 mmol/L, bicarbonate 18 mmol/L, albumin 2.8 g/dL, fasting glucose 168 mg/dL, Hb 9.4 g/dL. Urine dipstick: protein 3+, glucose 1+, blood negative, nitrite negative. A 24-hour urine collection contains 0.9 g creatinine and 3.2 g protein (normal protein < 0.15 g/day). Ultrasound shows both kidneys are small (right 8.1 cm, left 7.9 cm) with a thin cortex. He asks whether drinking 4 L of water a day will "wash out" the creatinine, whether a salt substitute and orange juice are good for him, and whether one dialysis session will cure him. He also mentions taking ibuprofen tablets twice daily for three months for knee pain.
1. Calculate his creatinine clearance from the 24-hour collection and interpret it. State one limitation of the calculation.
Read the worked answer
Clearance = amount of creatinine excreted per unit time ÷ plasma concentration.
- Excreted: 0.9 g/day = 900 mg/day.
- Plasma: 3.4 mg/dL = 34 mg/L.
- Clearance = 900 ÷ 34 = 26.5 L/day = 26 500 mL ÷ 1440 min = 18 mL/min.
Normal clearance is roughly 100–125 mL/min, so this is about 15% of normal, indicating severe impairment.
One limitation is that creatinine is not only filtered but also secreted by the tubules, so clearance slightly overestimates the true filtration rate. Another is collection completeness: 0.9 g of creatinine per day is low for a man, which suggests the 24-hour sample was under-collected; if so, the true value is somewhat higher than 18 mL/min but still far below normal. Repeated measurements and body-size correction would be needed before staging.
2. Using the data, explain the frothy urine, the ankle swelling, the high potassium and the acidosis. Why are the salt substitute and orange juice a danger?
Read the worked answer
Frothy urine is albuminuria: 3.2 g of protein a day is roughly twenty times normal, and albumin foams when urine is passed. Long-standing diabetes has damaged the glomerular filter, so protein leaks. Loss of albumin lowers plasma oncotic pressure, so fluid moves into the tissues and collects in the ankles; the low plasma albumin of 2.8 g/dL supports this. The Hb of 9.4 g/dL fits anaemia from reduced erythropoietin production by scarred kidneys.
Potassium 5.8 mmol/L is raised because fewer nephrons can excrete it and because acidosis shifts potassium out of cells; bicarbonate 18 mmol/L shows the kidney is failing to excrete acid. Both the salt substitute and orange juice are potassium-rich - substitutes are usually potassium chloride - so either could push potassium to a level that disturbs heart rhythm. Salt substitutes are safe for healthy people but are hazardous in kidney failure.
3. Evaluate the three things he asked about: 4 L of water, dialysis as a cure, and his ibuprofen. What advice follows?
Read the worked answer
Drinking 4 L will not wash out creatinine, because the problem is filtration, not flushing; the kidney cannot excrete the solute whatever the volume. If his urine output is reduced, a large intake risks fluid overload and dilution of sodium, so fluid allowance should be matched to urine output plus insensible losses rather than set at a fixed 4 L.
Dialysis is a replacement, not a cure. It removes urea, creatinine, potassium and excess water across a semipermeable membrane by diffusion and ultrafiltration, but it does not restore erythropoietin, vitamin D activation or tubular reabsorption. Small kidneys with a thin cortex show chronic, irreversible scarring, so function will not return.
Ibuprofen is likely to have caused a further acute fall in function: NSAIDs block prostaglandins that keep the afferent arteriole open, and a kidney already dependent on that compensation is vulnerable. He should stop it and use alternative pain relief. He needs nephrology referral, blood pressure and glucose control, dietary protein and potassium restriction, review of all drugs, and planning for renal replacement therapy.