Anaemia is common in chronic renal impairment, but not in acute renal failure, and so its presence can help distinguish between the two (although, taken in isolation, it isn't reliable).
The pathogenesis is meant to be a combination of:
- Decreased erythropoietin secretion - the kidney is responsible for this hormone's production, remember? Without adequate secretion, the bone marrow doesn't get the message to make more red blood cells.
- Bone marrow insensitivity to erythropoietin - it seems that amongst the miscellaneous "toxins" that build up when the kidneys aren't working are substances the depress bone marrow production.
Clearly the first mechanism is more easy (if expensive) to correct than the second. Simply by giving patients with chronic kidney disease some erythropoeitin, one can alleviate much of the anaemia. This is often employed in such patients.