A 19-year-old man is evaluated for proteinuria discovered during a routine sports preparticipation evaluation. He has been asymptomatic, has no other medical problems, and takes no medications.
On physical examination, temperature is 36.6°C (97.8°F) blood pressure is 110/72 mm Hg, pulse rate is 60/min, and respiration rate is 12/min. BMI is 18. The funduscopic, cardiopulmonary, and skin findings are normal. No peripheral edema is present.
||15 mg/dL (150 g/L)
||4.1 g/dL (41 g/L)
||130 mg/dL (3.4 mmol/L)
||0.7 mg/dL (61.9 µmol/L)
||92 mg/dL (5.1 mmol/L)
||6.5 g/dL (65 g/L)
|Spot urine protein–creatinine ratio
||Urine dipstick: protein, 2+; blood, negative; glucose, negative. Microscopic: rare hyaline cast, no cells or crystals.
The most appropriate next diagnostic test is to evaluate the patient for orthostatic proteinuria. This is done by obtaining separate upright (daytime) and supine (overnight) urine collections for protein quantitation. Orthostatic proteinuria is defined by an increase in urinary protein excretion only in the upright position; when supine, the urinary protein excretion rate is normal (<50 mg/8 h). The condition is seen most commonly in children or young adults. The total urine protein excretion rate is usually less than 1 g/24 h, and the urinalysis is otherwise normal. The etiology of the condition is uncertain; in some cases, mild alterations in glomerular histology have been reported. An association of orthostatic proteinuria with entrapment of the left renal vein between the aorta and the superior mesenteric artery (“nutcracker syndrome”) also has been reported. The condition is benign; it often resolves spontaneously, and long-term follow-up studies in affected patients have shown renal function remains normal.
A kidney biopsy is not the most appropriate next step in this patient's management. His medical history provides no evidence of significant past or present illness, and his physical examination and initial laboratory studies do not suggest serious kidney disease. Specifically, he is normotensive and not edematous; the serum creatinine, albumin, and cholesterol are normal; and, the urinalysis is normal.
A single 24-hour urine collection would most likely only confirm the subnephrotic rate of urinary protein excretion, which is already evident from the spot urine protein–creatinine ratio of 0.8 mg/mg. More importantly, a single 24-hour urine collection would fail to differentiate the rates of upright versus supine proteinuria, which is essential in distinguishing orthostatic from persistent proteinuria.
Given the patient's age, normal hemoglobin, and serum protein, a dysproteinemia is very unlikely, and serum and urine protein electrophoresis studies are not needed. Should the patient prove to have persistent proteinuria, urine protein electrophoresis and immunofixation studies may be useful in determining whether the proteinuria reflects glomerular or tubular disease.
- Orthostatic proteinuria is defined by an increase in urinary protein excretion only in the upright position.