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

Given the patient's symptoms and urinalysis results, the differential diagnosis can be categorized as follows:

• Single most likely diagnosis:

  • Acute Interstitial Nephritis (AIN): This condition is often associated with the use of nonsteroidal anti-inflammatory drugs (NSAIDs), which the patient has been taking in large doses. The presence of a skin rash, decreased urine volume, and the specific urinalysis findings (e.g., proteinuria, hematuria, leukocyturia, and the presence of WBC casts) support this diagnosis. AIN can lead to acute kidney injury, which aligns with the patient's decreased urine output and other symptoms.

• Other Likely diagnoses:

  • Acute Tubular Necrosis (ATN): Although less directly linked to NSAID use compared to AIN, ATN can result from various insults, including medications. The patient's decreased urine volume and abnormal urinalysis results could also suggest ATN, especially if there were episodes of hypovolemia or direct nephrotoxic effects from the medication.
  • Nephrotic Syndrome: The significant proteinuria (3+) could suggest nephrotic syndrome, but the presence of hematuria, leukocyturia, and WBC casts makes this less likely as a primary diagnosis. Nephrotic syndrome is more typically characterized by heavy proteinuria with minimal hematuria or leukocyturia.

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

  • Rapidly Progressive Glomerulonephritis (RPGN): Although less common, RPGN can present with hematuria, proteinuria, and a rapid decline in renal function. It is crucial to consider RPGN due to its potential for severe and irreversible kidney damage if not promptly treated.
  • Sepsis-related Acute Kidney Injury (AKI): The presence of leukocyturia and WBC casts could also indicate a systemic infection leading to sepsis and subsequent AKI. Given the potential for high mortality, sepsis must be considered and ruled out.

• Rare diagnoses:

  • Vasculitis: Conditions like ANCA-associated vasculitis could present with renal involvement, including hematuria and proteinuria, along with systemic symptoms. However, the absence of other typical symptoms (e.g., respiratory or gastrointestinal involvement) makes this less likely.
  • Tubulointerstitial Nephritis with Uveitis (TINU) Syndrome: This rare condition could present with interstitial nephritis and uveitis, but it is much less common and would require specific diagnostic criteria to be considered.

Additional Considerations

• a. Most diagnostic reagent strip result: The presence of blood and protein in significant amounts, along with leukocyte esterase, are highly suggestive of an inflammatory process affecting the kidneys.
• b. Correlation with microscopic results: These findings correlate with the microscopic presence of RBCs, WBCs, WBC casts, and hyaline casts, indicating an active inflammatory and possibly nephritic process.
• c. Additional test (stain) might be requested: A renal biopsy might be considered for definitive diagnosis, especially if the patient's condition does not improve with withdrawal of the offending agent or if there's a need to differentiate between various causes of acute kidney injury.
• d. Most probable diagnosis: Based on the information provided, Acute Interstitial Nephritis (AIN) due to NSAID use is the most probable diagnosis, given the clinical context and urinalysis findings.