Differential Diagnosis for the 40-year-old Female Tennis Player

Single most likely diagnosis

• Acute Interstitial Nephritis (AIN): This condition is likely given the patient's recent use of nonsteroidal anti-inflammatory agents (NSAIDs), which are known to cause AIN. The presence of a skin rash, decreased urine volume, and the urinalysis results (e.g., proteinuria, hematuria, leukocyturia, and the presence of WBC casts) support this diagnosis. AIN can lead to acute kidney injury, which explains the decreased urine volume and the abnormal urinalysis findings.

Other Likely diagnoses

• Acute Tubular Necrosis (ATN): Although less likely than AIN given the context, ATN could be considered due to the patient's use of NSAIDs, which can also cause ATN, especially with prolonged use or in high doses. However, the presence of WBC casts and a skin rash more strongly suggests AIN.
• Nephrotic Syndrome: The significant proteinuria (3+) could suggest nephrotic syndrome, but the presence of hematuria, leukocyturia, and WBC casts points more towards an inflammatory process like AIN rather than a primary glomerular disease.

Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)

• Severe Pyelonephritis or Sepsis: Although the nitrite test is negative and there's no clear indication of a urinary tract infection (UTI) being the primary issue, severe pyelonephritis or sepsis could present with similar symptoms and urinalysis findings. It's crucial to consider and rule out these conditions due to their potential severity and need for immediate treatment.
• Rapidly Progressive Glomerulonephritis (RPGN): This condition can present with hematuria, proteinuria, and a rapid decline in renal function. While less likely, it's a critical diagnosis not to miss due to its potential for severe outcomes if not promptly treated.

Rare diagnoses

• Vasculitis (e.g., ANCA-associated vasculitis): Although rare, vasculitis could present with renal involvement (manifesting as hematuria and proteinuria), skin rash, and systemic symptoms. The presence of WBC casts could also be seen in vasculitis affecting the kidneys.
• Tubulointerstitial Nephritis with Uveitis (TINU syndrome): This is a rare condition that could present with interstitial nephritis and uveitis, among other symptoms. It's less likely but should be considered in the differential diagnosis, especially if other systemic symptoms are present.

Answer to the specific questions:

a. The most diagnostic reagent strip result helping to rule out UTI is the negative nitrite test, as most UTIs caused by common pathogens (e.g., E. coli) would produce a positive nitrite test.

b. The negative nitrite test correlates with the microscopic results (presence of WBC casts, significant leukocyturia, and hematuria) by suggesting that the inflammation is not primarily due to a bacterial infection (which would likely be positive for nitrite) but rather an inflammatory process like AIN.

c. An additional test that might be requested on this specimen is a renal biopsy, which could provide a definitive diagnosis of the underlying condition affecting the kidneys, such as AIN, ATN, or other glomerular diseases.

d. The most probable diagnosis, as discussed, is Acute Interstitial Nephritis (AIN) due to the patient's use of NSAIDs, the presence of a skin rash, and the specific findings on urinalysis.