Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (524 page)

   Urine osmolality is typically below 400 mOsm/kg.

   Fractional excretion of sodium (FENa) is an accurate test to differentiate between prerenal disease (<1%) and ATN (>1%). There are few limitations, however, to the use of FENa in determining the cause of AKI since it may be <1% in some ATN cases (e.g., when ATN is associated with a chronic prerenal disease such as heart failure), or >1% in some prerenal disease cases (e.g., patients treated with diuretics).

   Sudden elevation in serum creatinine with normal BUN/creatinine ratio (10–15:1).

CHRONIC KIDNEY DISEASE

   Definition

   Chronic kidney disease (CKD) occurs when there is a progressive, irreversible alteration in kidney structure and function, as reflected by a gradual decrease in GFR and increase in BUN and creatinine, and/or albuminuria. This condition becomes more prevalent with increasing age.

   CKD is defined by the Kidney Disease Outcomes Quality Initiative (KDOQI) as

   Kidney damage for ≥3 months as defined by structural or functional abnormalities of the kidney, with or without decreased GFR, manifested by either pathologic abnormalities or markers of kidney damage

   GFR <60 mL/minute/1.73 m
²
for ≥3 months, with or without kidney damage

   CKD is divided into five stages based on GFR (see Table
12-3
). Stage 5 or kidney failure is the most advanced stage. The term end-stage renal disease (ESRS) refers to chronic kidney failure treated with either dialysis or transplantation.

   CKD is usually asymptomatic in its early stages (1–3). Symptoms and metabolic complications (e.g., anemia, hyperparathyroidism, water and electrolyte imbalance) usually appear at later stages when GFR drops below 30 mL/ minute/1.73 m
²
.

   Another staging system uses urine albumin-to-creatinine ratio (mg/g): stage 1: <30 mg/g; stage 2: 30–299 mg/g; stage 3 ≥ 300 mg/g.

   The distinction between acute, subacute, and chronic kidney disease is not always well defined but may be important, since AKI may be reversible, whereas CKD is not. A reduced size of the kidney (demonstrated by ultrasound) indicates a chronic phase.