CKD — MRCS Part B Applied surgical science and critical care

A renal station that begins with an acidotic gas and a set of bloods, then moves from acute kidney injury to chronic kidney disease and the physiology of measuring what the kidney does.

Reading the results

A raised creatinine and urea with hyperkalaemia and a metabolic acidosis with a low bicarbonate is the biochemistry of renal failure; the gas shows the acidosis and the respiratory compensation. Acute kidney injury is managed by treating the threat first, hyperkalaemia with calcium gluconate, insulin and glucose and salbutamol, then finding and reversing the cause: restore the circulating volume if it is pre-renal, relieve any obstruction, stop nephrotoxic drugs (NSAIDs, ACE inhibitors, aminoglycosides, contrast), treat sepsis, monitor the urine output with a catheter and the electrolytes daily, and refer for renal replacement if the potassium, acidosis, fluid overload or uraemia cannot be controlled. Most acute injury from hypoperfusion recovers within days to weeks once the cause is treated; recovery from acute tubular necrosis takes longer, and a proportion, larger with each episode and in the elderly, are left with chronic disease or on dialysis. Mortality in the intensive care patient who needs dialysis is high, mostly from the underlying illness.

Creatinine and urea

Serum creatinine is normally about 60–110 micromol/L in a man and 45–90 in a woman, depending on muscle mass. It comes from muscle at a steady rate, is freely filtered, and is barely reabsorbed, so its level reflects glomerular filtration and little else. Urea is made in the liver from protein, so it rises with a high-protein diet, gastrointestinal bleeding, catabolism and steroids, and falls with liver disease; and about half of what is filtered is reabsorbed, more so when the flow is slow, so it rises out of proportion in dehydration. That makes the urea-to-creatinine ratio a useful clue to a pre-renal cause, but creatinine the better measure of function.

Chronic kidney disease

Abnormal kidney structure or function present for more than three months, staged by the estimated GFR: stage 1 above 90 with other evidence of damage, 2 from 60 to 89, 3a and 3b from 45 to 59 and 30 to 44, 4 from 15 to 29, and 5 below 15, which is end-stage disease. Causes: diabetes and hypertension above all, then glomerulonephritis, polycystic kidneys, chronic pyelonephritis and reflux, obstruction, renovascular disease, drugs and myeloma. Complications: anaemia from lost erythropoietin, renal bone disease from phosphate retention and lost vitamin D activation with secondary hyperparathyroidism, hypertension and fluid overload, hyperkalaemia and acidosis, a greatly increased cardiovascular risk, pericarditis and neuropathy in uraemia, and a bleeding tendency from platelet dysfunction that matters before surgery.

What the urinary system does

Excretes the water-soluble waste of metabolism, above all urea and creatinine; regulates the volume and osmolality of the body fluids and the balance of sodium, potassium, calcium and phosphate; regulates acid-base balance by excreting hydrogen ions and reclaiming bicarbonate; makes erythropoietin and renin and activates vitamin D; and stores and voids urine.

Clearance and GFR

The renal clearance of a substance is the volume of plasma cleared of it completely per unit time, calculated as urine concentration times urine flow rate divided by plasma concentration. For a substance that is freely filtered and neither reabsorbed nor secreted, the amount cleared equals the amount filtered, so its clearance equals the glomerular filtration rate. Inulin has those properties, and is not made or broken down by the body, but it has to be infused to a steady state and measured in timed urine collections, so it is a research tool. In practice GFR is estimated from creatinine: a creatinine clearance from a 24-hour urine collection, or, far more often, an equation (CKD-EPI, or MDRD before it) that estimates it from the serum creatinine, age, sex and ethnicity. Creatinine is secreted a little by the tubules, so its clearance slightly overestimates GFR. Normal creatinine clearance is about 90–140 ml/min, roughly 125 ml/min in a young adult, and it falls by about 1 ml/min a year after the age of 40.