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Difficulty: MediumFormulating a Differential Diagnosis

A 24-year-old woman comes to the clinic complaining of a 4-day history of painful urination and increased urinary frequency. She has not experienced fever, chills, back pain, or gross hematuria. Her medical history is unremarkable, and she takes no medications other than an oral contraceptive. She reports having a new male sexual partner for the past month. Vital signs are normal. Physical examination shows no costovertebral angle tenderness, and a pelvic examination reveals no vaginal discharge or cervical motion tenderness. Initial urinalysis in the clinic shows 15-20 WBCs/hpf, positive leukocyte esterase, and negative nitrites. She is empirically prescribed a 5-day course of nitrofurantoin for presumed uncomplicated cystitis. Three days later, she reports no improvement in her symptoms. The standard urine culture from her initial visit is finalized as 'no growth' at 48 hours. Which of the following is the most likely diagnosis?

  1. Chlamydia trachomatis urethritisAnswer
  2. B
    Enterococcus faecalis cystitis
  3. C
    Interstitial cystitis
  4. D
    Distal ureterolithiasis
  5. E
    Chemical urethritis

Answer

Chlamydia trachomatis urethritis
The correct diagnosis is Chlamydia trachomatis urethritis. This patient presents with acute dysuria, frequency, and pyuria (elevated WBCs) but has a negative standard urine culture. This classic triad is known as 'sterile pyuria'. In a young, sexually active female with a new partner, sterile pyuria is highly suspicious for a sexually transmitted infection causing urethritis, such as Chlamydia trachomatis or Neisseria gonorrhoeae. These atypical intracellular pathogens do not grow on standard agar used for routine urine cultures. Furthermore, Chlamydia does not respond to nitrofurantoin, which explains her treatment failure. Diagnosis requires a nucleic acid amplification test (NAAT).

Step-by-Step Solution

1
Analyze the urinalysis and culture results in the context of the patient's symptoms.
The patient has symptomatic pyuria (positive leukocyte esterase, 15-20 WBCs) but a negative standard urine culture, which characterizes 'sterile pyuria'.
Standard urine cultures readily grow common uropathogens like E. coli and Enterococcus, but fail to grow atypical or intracellular organisms.
2
Evaluate the clinical risk factors and the response to initial empiric treatment.
The patient has a new sexual partner and failed empiric therapy with nitrofurantoin.
Nitrofurantoin treats common bacterial cystitis but is ineffective against intracellular organisms like Chlamydia. The new sexual partner significantly increases the pre-test probability of a sexually transmitted infection.
3
Synthesize findings to formulate the most likely differential diagnosis.
Sterile pyuria in a young, sexually active female with dysuria strongly points to Chlamydial or Gonococcal urethritis.
These pathogens require nucleic acid amplification testing (NAAT) for accurate diagnosis and specific antibiotics (e.g., doxycycline or azithromycin) for definitive treatment.

Key Concept

Sterile pyuria (dysuria and WBCs in urine with no growth on a standard culture) in a sexually active young adult is highly suggestive of atypical urethritis, most commonly caused by Chlamydia trachomatis or Neisseria gonorrhoeae.
Estimated Time:1m 30s
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Formulating a Differential Diagnosis

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