Noncontrast helical abdominal CT is the most appropriate diagnostic test to perform next in this patient with a clinical presentation consistent with nephrolithiasis. The findings of unilateral pain combined with hematuria without inflammation on urinalysis suggest nephrolithiasis, and the location of the pain in the inguinal region suggests that the stone may be in the distal ureter. Ultrasonography is increasingly used as an initial study for evaluation of suspected nephrolithiasis because of increased availability, lack of radiation exposure, and lower cost than CT; it is also the study of choice in pregnant patients. However, ultrasonography is less sensitive than CT for detecting kidney stones in the distal ureter or for evaluating other potential nonurologic conditions that may be responsible for the pain. Given this patient's clinical picture that is consistent with nephrolithiasis but with a negative ultrasound for kidney stones, further imaging with noncontrast helical abdominal CT is indicated. Additionally, the absence of hydronephrosis on ultrasound does not rule out nephrolithiasis.

Although renal vein thrombosis can cause hematuria, this diagnosis is less likely given the location of this patient's pain, normal kidney function, and lack of proteinuria. Therefore, Doppler ultrasonography of the renal veins is inappropriate.

Kidney biopsy may be appropriate for patients with suspected glomerulonephritis. Glomerulonephritis typically presents with evidence of decreased kidney function with inflammation and glomerular damage seen as variable proteinuria, hematuria, and possibly dysmorphic erythrocytes and erythrocyte casts on urinalysis. However, this patient's clinical history and laboratory findings are not consistent with glomerulonephritis, and kidney biopsy is not indicated.

Urine cultures are appropriate to diagnose a urinary tract infection or pyelonephritis. However, a urinary tract infection is unlikely in this patient given the absence of dysuria and a urinalysis negative for significant leukocytes, leukocyte esterase, or nitrites.