Assessment of Glomerular Filtration Rate (GFR)GFR is the primary metric for kidney "function," and itsdirect measurement involves administration of a radioactiveisotope (such as inulin or iothalamate) that is filtered at theglomerulus but neither reabsorbed nor secreted throughoutthe tubule. Clearance of inulin or iothalamate in millilitersper minute equals the GFR and is calculated from the rateof removal from the blood and appearance in the urine overseveral hours. Direct GFR measurements are frequentlyavailable through nuclear radiology departments. In mostclinical circumstances direct measurement of GFR is notavailable, and the serum creatinine level is used as asurrogate to estimate GFR.
• Cockcroft-Gault: CrCl (mL/min) = (140 – age (years) x weight (kg) x [0.85 if female])/(72 x sCr (mg/dL)• MDRD: eGFR (mL/min per 1.73 m2) = 186.3 x PCr (e–1.154) x age (e–0.203) x (0.742 if female) x (1.21 if black).• CKD-EPI: eGFR = 141 x min (Scr/k, 1)a x max (Scr/k, 1)–1.209 x 0.993Age x 1.018 [if female] x 1.159 [if black]• where Scr is serum creatinine, k is 0.7 for females and 0.9 for males, a is –0.329 for females and – 0.411 for males, min indicates the minimum of Scr/k or 1, and max indicates the maximum of Scr/k or 1
Approach to the PatientOnce it has been established that GFR is reduced, thephysician must decide if this represents acute or chronicrenal injury. The clinical situation, history, and laboratorydata often make this an easy distinction. However, thelaboratory abnormalities characteristic of chronic renalfailure, including anemia, hypocalcemia, andhyperphosphatemia, often are also present in patientspresenting with acute renal failure. Radiographic evidenceof renal osteodystrophy can be seen only in chronic renalfailure but is a very late finding, and these patients areusually on dialysis. The urinalysis and renal ultrasoundoccasionally can facilitate distinguishing acute from chronicrenal failure.
Prerenal FailureThe etiologies of prerenal azotemia include anycause of decreased circulating blood volume(gastrointestinal hemorrhage, burns, diarrhea,diuretics), volume sequestration (pancreatitis,peritonitis, rhabdomyolysis), or decreased effectivearterial volume (cardiogenic shock, sepsis). Renalperfusion also can be affected by reductions incardiac output from peripheral vasodilation (sepsis,drugs) or profound renal vasoconstriction [severeheart failure, hepatorenal syndrome, drugs such asnonsteroidal anti-inflammatory drugs (NSAIDs)].
Postrenal AzotemiaUrinary tract obstruction accounts for <5% of cases ofacute renal failure, but it is usually reversible and must beruled out early in the evaluation. Since a single kidney iscapable of adequate clearance, obstructive acute renalfailure requires obstruction at the urethra or bladder outlet,bilateral ureteral obstruction, or unilateral obstruction in apatient with a single functioning kidney. Obstruction usuallyis diagnosed by the presence of ureteral and renal pelvicdilation on renal ultrasound. However, early in the course ofobstruction or if the ureters are unable to dilate (e.g.,encasement by pelvic tumors or periureteral), theultrasound examination may be negative.
Intrinsic Renal DiseaseIschemic and toxic ATN account for 90% of casesof acute intrinsic renal failure. The clinical settingand urinalysis are helpful in separating thepossible etiologies of acute intrinsic renal failure.Prerenal azotemia and ATN are part of a spectrumof renal hypoperfusion; evidence of structuraltubule injury is present in ATN, whereas promptreversibility occurs with prerenal azotemia uponrestoration of adequate renal perfusion. Thus, ATNoften can be distinguished from prerenal azotemiaby urinalysis and urine electrolyte composition.
Oliguria and AnuriaOliguria refers to a 24-h urine output <400 mL, and anuriais the complete absence of urine formation (<100 mL).Anuria can be caused by total urinary tract obstruction, totalrenal artery or vein occlusion, and shock (manifested bysevere hypotension and intense renal vasoconstriction).Cortical necrosis, ATN, and rapidly progressiveglomerulonephritis occasionally cause anuria. Oliguria canaccompany any cause of acute renal failure and carries amore serious prognosis for renal recovery in all conditionsexcept prerenal azotemia. Nonoliguria refers to urine output>400 mL/d in patients with acute or chronic azotemia.