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Hematuria: Glomerular versus extraglomerular bleeding

INTRODUCTION

One of the first steps in the evaluation of the patient with hematuria is to try to distinguish between glomerular and extraglomerular bleeding. The degree of bleeding (gross versus microscopic) is not helpful, but careful evaluation of the urine may help to establish the correct diagnosis. The importance of this distinction is that the evaluation of glomerular and extraglomerular bleeding is markedly different. (See "Evaluation of hematuria in adults".)

URINARY FINDINGS

Among patients with hematuria, a variety of findings on the urinalysis favor the diagnosis of glomerular bleeding. These include the presence of red cell casts, proteinuria, dysmorphic red cells (particularly acanthocytes), and, in patients with gross hematuria, a smoky brown color (table 1). On the other hand, blood clots are almost always indicative of extraglomerular bleeding.

Red cell casts — The presence of red cell casts is virtually diagnostic of glomerulonephritis or vasculitis, although such casts are infrequently seen in acute interstitial nephritis (picture 1) [1]. The absence of these casts, however, does not exclude glomerular disease.

Red cell casts tend to accumulate at the edges of the coverslip. Thus, one has to examine all of the microscopic fields, initially at low power. Prolonged centrifugation may disrupt cellular casts, diminishing the likelihood of identifying such casts.

Proteinuria — Protein excretion above 500 mg/day is strongly suggestive of a glomerular lesion, because hematuria alone does not lead to a significant increase in proteinuria. As an example, as little as 1 mL of blood in one liter of urine can produce a visible color change. This quantity of blood contains approximately 0.6 mL of plasma, which will contain only 35 mg of protein at a plasma protein concentration of 6 g/dL (60 g/L). A protein concentration of 35 mg/L is below the sensitivity of the urine dipstick for protein and therefore will not be detected on routine urine examination. (See "Measurement of urinary protein excretion".)
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References Top
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