Approach to renal failure

Today we discussed acute renal failure. Some important points:

The time-tested approach to acute renal failure (or 'acute kidney injury') is
Prerenal, renal, and post-renal

Prerenal:

Hypovolemia: low intake, GI, GU, or insensible losses, hemorrhage, vasodilation)
Low effective circulating volume: CHF, cirrhosis, 'third spacing')
Renal vasoconstrictors: GN sepsis, cyclosporine, NSAIDS, ACE-I/ARB (sort of; different mechanism), hepatorenal syndrome
Characterized by clinical signs of hypovolemia or above problems, bland urine, urine osmol over 450, low urine Na. Responds to fluid replacement

Renal parenchymal:

1) Glomerular
Look for nephritic syndrome: HTN, rbcs, rbc casts, sub-nephrotic proteinuria
or nephrotic syndrome: edema, dyslipidemia, heavy proteinuria (over 3 g/day), hypoalbuminemia, thrombosis
Rapidly Progressive GN AKA 'crescentic' GN: mainly present as nephritic, but sometimes nephrotic (or both!)
1. Immune complexes: (with decreased complement- post strep, endocarditis, SLE, cryoglobulinemia. With normal complement levels- endocarditis, IgA)
2. Linear (Goodpasture's, AGBM)
3. Pauci-immune (Wegener's, Churg-Strauss, microscopic polyangiitis)

Other mainly nephritic: IgA nephropathy (can also be nephrotic), membranoproliferative GN, focal proliferative GN

Mainly nephrotic: minimal change, FSGS, membranous, nodular

2) Tubulointerstitial:
AIN: Drugs: Abx (methicillin classic, also quinolones), NSAID, allopurinol. Infection- pyelo, CMV, legionella. Neoplastic: myeloma, amyloid. May see WBC casts (shown above), urine eosinophils (Sn ~70%. Sp ~80%)
ATN: ischemic (hypotension, sepsis), toxic: exogenous (meds- aminoglycosides, ampho, cisplatin, contrast) or endogenous (myoglobin, hemoglobin, uric acid eg. from tumor lysis, Ca2+). Hallmark is heme granular casts. Characterized by high urine Na, low osmolality.

3) Vascular:
Large vessel: RAS RA thrombosis vasculitisrenal v. thrombosis esp. in nephrotic syn
Small vessel: HTN nephrosclerosis, scleroderma renal crisis, cholesterol embolic (livedo reticularis, blue toes, dusky feet, eosinophilia) TTP/HUS, antiphospholipid syndrome, DIC

Postrenal:

Lower UT obstruction (prostate, mass, etc) or effective obstruction (e.g. retention from meds), neurogenic bladder, UTI, bladder stones, bilateral stones or stone with 1 kidney, retroperitoneal fibrosis (NB- may have no hydronephrosis here), lymphoma
Usually characterized by bland urine, imaging evidence of obstruction (but not always). Urine output is not always low.

General approach:

1. Previous creatinine
2. Hx: drugs, volume, systemic disease (lung, joint, skin, fever...)
3. Volume status assessment, assesment for dialysis indications
4. Urinalysis: blood, protein, casts, cells, eosinophils
5. Urine lytes, osmolality
6. U/S of abdo to r/o obstruction
7. Serology: ANA, anti-GBM, ANCA, C3, C4.

Link:

Click here for a NEJM CPC that goes over the approach to ARF