LABORATORY EVALUATION

In hospitalized adults in whom the diagnoses of prerenal and postrenal azotemia have been excluded, ARF is usually caused by ATN. By contrast, in an outpatient setting in which prerenal and postrenal causes have been excluded, ARF is more often caused by other renal parenchymal diseases.

Examination of the urine for blood and protein and of the urine sediment can give valuable information that often helps to narrow considerably the diagnostic possibilities and to suggest further appropriate laboratory evaluation.

The presence of 3+ to 4+ protein, 2+ to 3+ blood, and an active sediment with red blood cells(RBCs) and RBC casts is characteristic of proliferative glomerulonephritis. A history of an underlying disease such as systemic lupus erythematosus, complement levels, antinuclear factor, and kidney biopsy(if the kidney size is normal) generally helps to clarify the diagnosis. The presence of only a few RBCs in the urine sedi ment with a strongly heme-positive urine or a hemepositive supernatant(with the RBCs removed by centrifugation) most commonly results from myoglobinuria or hemoglobinuria. Patients with rhabdomyolysis have a marked increase in the muscle enzymes such as creatinine phosphokinase. The urine sediment in patients with myoglobinuria may show RBCs, pigmented casts granular casts, and numerous uric acid crystals. Kidney size gives important clues about whether the renal failure is chronic and whether obstruction is present. ultrasonography is the initial procedure of choice because it is noninvasive and reliable. The finding of normal-sized kidneys in a patient with advanced azotemia generally suggests that the patient has acute rather than chronic renal failure; however, several important causes of chronic renal failure, including diabetes mellitus, multiple myeloma, and amyloidosis, may be associated with norma-sized kidneys.

The renal ultrasound examination is also helpful in(1) making a diagnosis of polycystic kidney disease; (2) determining whether one or two kidneys are present; and(3) localizing the kidney for renal biopsy. Normal kidney size in a patient with renal failure is often an indication for renal biopsy.

Before a renal biopsy is carried out, the patient’s blood pressure must be controlled, bleeding and coagulation parameters must be checked, and the presence of two kidneys must be confirmed.

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