How does taking AZO (phenazopyridine) prior to a urine screen affect the results, particularly for a patient with suspected urinary tract infection (UTI)?

Effect of Phenazopyridine (AZO) on Urine Screen Results

Phenazopyridine does not interfere with the chemical or microscopic components of urinalysis that are used to diagnose UTI, but it can cause orange-red discoloration of urine that may complicate visual interpretation and mask hematuria. 1

Impact on Urinalysis Components

Chemical Dipstick Testing

• Leukocyte esterase and nitrite tests remain accurate despite phenazopyridine use, as the drug does not chemically interfere with these enzymatic reactions 2
• The orange-red discoloration from phenazopyridine is a visual change only and does not affect the biochemical detection of pyuria or bacteriuria 3
• Automated urinalysis readers can still accurately detect leukocyte esterase (sensitivity 83%, specificity 78%) and nitrite (specificity 98-100%) in the presence of phenazopyridine 2

Microscopic Examination

• Microscopic analysis for white blood cells, bacteria, and epithelial cells is unaffected by phenazopyridine, as the drug does not alter cellular morphology or bacterial presence 2
• The threshold of ≥10 WBCs/high-power field for pyuria remains valid for diagnosis regardless of phenazopyridine use 2

Urine Culture

• Phenazopyridine has no antibacterial properties and therefore does not interfere with bacterial growth on culture media 1
• Culture results remain accurate and reliable for identifying pathogens and determining antimicrobial susceptibilities 1

Critical Clinical Pitfall

The most important concern is not test interference, but rather that patients may use phenazopyridine as monotherapy for UTI symptoms, delaying appropriate antibiotic treatment. 1

• A case report documented progression from uncomplicated cystitis to acute pyelonephritis when a patient relied solely on phenazopyridine for symptom relief without antibiotics 1
• The analgesic effect of phenazopyridine can mask worsening symptoms, giving false reassurance while the infection ascends to the kidneys 1
• Phenazopyridine should only be used as adjunctive therapy alongside appropriate antibiotics, never as monotherapy 4

Proper Clinical Approach

When Phenazopyridine Has Been Used

• Proceed with standard urinalysis interpretation using leukocyte esterase, nitrite, and microscopic examination 2
• Obtain urine culture before initiating antibiotics if the patient has not yet started antimicrobial therapy 5
• If the patient has been using phenazopyridine alone for >48 hours without antibiotics and symptoms persist or worsen, consider the possibility of ascending infection 1

Diagnostic Algorithm

• If urinalysis shows pyuria (≥10 WBCs/HPF or positive leukocyte esterase) AND the patient has UTI symptoms (dysuria, frequency, urgency, fever, hematuria), initiate antibiotic therapy regardless of phenazopyridine use 2
• The combination of positive leukocyte esterase and nitrite achieves 93% sensitivity and 96% specificity for UTI, unaffected by phenazopyridine 2
• Negative leukocyte esterase and nitrite effectively rule out UTI (negative predictive value 82-91%), even in patients taking phenazopyridine 2

Patient Education Imperative

• Counsel patients that phenazopyridine only treats symptoms, not the infection itself 1
• Emphasize that antibiotics are required to eradicate the bacterial infection and prevent complications 1
• Advise limiting phenazopyridine use to 2 days maximum as adjunctive therapy alongside antibiotics 4