How do I tell the difference between acute and chronic renal failure?

If you haven't yet encountered this problem, you are obviously fairly new to the medicine thing! The picture is common: bloodwork reveals that the patient's urea (BUN) and creatinine levels are elevated, which almost always signifies renal dysfunction. But is this an acute or a chronic problem.

Sometimes, a history will help in this regard. For instance, the patient may inform you that she comes in for dialysis every second week, in which case you should be shot for thinking her renal failure to be acute!

More often, though, the history isn't conclusive. Thankfully, there are a few other ways of telling:

• Small kidneys indicate chronic renal failure. (Think of it as an old, scarred set of kidneys - and recall that scars contract after a while.) This is usually checked by ultrasound.
• A normocytic anaemia also points towards the problem being chronic. Erythropoietin, the hormone that signals the bone marrow to produce red blood cells, is produced in the kidney. Although both acute and chronic kidney dysfunction will result in decreased erythopoietin levels, the actual haemoglobin levels will only have time to decline appreciably in the latter instance.
• Low urine volumes are more characteristic of acute renal failure than its chronic counterpart, although there are exceptions. In chronic renal failure, the tubules are proportionately more inadequate in reabsorbing fluid than the glomeruli are at filtering it, and so polyuria results. By contrast, in acute renal failure the glomeruli usually shut down almost completely. (However, in the classical 'recovery phase', polyuria results from the filtering of all the osmolytes that had been obtained.)
• An otherwise healthy-looking patient with elevated urea and creatinine levels is very unlikely to be suffering from acute renal failure - the problem is much more likely to be chronic.

If in doubt, the answer will become obvious shortly. Chronic renal dysfunction won't change much, wheras acute renal failure will result in rapidly worsening (and then hopefully fairly rapidly improving) renal function.