Nephrology
Acute kidney injury
Divide into prerenal, intrinsic, and postrenal causes; obtain a bladder scan to exclude obstruction. Prerenal states show a fractional excretion of sodium below 1% and bland urine sediment (hepatorenal and contrast-associated injury are exceptions). Acute tubular necrosis produces muddy brown casts with FENa above 2%. Acute interstitial nephritis suggests fever, rash, eosinophilia, and white cell casts after a new drug - stop the culprit. Glomerulonephritis shows dysmorphic red cells and red cell casts; check complement, ANCA, and anti-GBM.
CKD and dialysis
Stage by GFR and albuminuria together. Manage blood pressure with ACE inhibitors or ARBs when proteinuria is present (never combine the two), treat acidosis with bicarbonate, monitor potassium and phosphate, and look for anemia from erythropoietin deficiency. Urgent dialysis indications: refractory acidosis, electrolytes (potassium), intoxication, volume overload, and uremia.
Acid-base and electrolytes
Calculate the anion gap and correct it for albumin. In metabolic acidosis, use Winter's formula to check whether respiratory compensation is appropriate; a delta-delta ratio outside 1–2 implies a mixed disorder. For hyponatremia, first measure serum osmolality to exclude hyperglycemia and pseudohyponatremia, then assess volume status. Correct sodium slowly - no more than 8–10 mEq/L per day.
Worked example
An elderly patient with vomiting has creatinine 2.0 and FENa 0.4% with bland urine. This is prerenal: give isotonic fluid, stop diuretics and NSAIDs, and recheck.