Can Pyridium (phenazopyridine) cause abnormal urine dipstick test results?

Pyridium (Phenazopyridine) Effects on Urinalysis Results

Pyridium (phenazopyridine) causes false-positive results on multiple urinalysis parameters due to its reddish-orange dye properties, which can interfere with colorimetric and spectrometry-based urinalysis tests. 1

How Phenazopyridine Affects Urinalysis

Phenazopyridine is an azo dye that functions as a urinary tract analgesic, providing symptomatic relief for conditions like urinary tract infections. However, it significantly impacts urinalysis results in several ways:

Confirmed Interferences:

• Urine Color: Produces a characteristic reddish-orange discoloration of urine 1
• Colorimetric Tests: Interferes with urinalysis based on spectrometry or color reactions 1
• Dipstick Parameters: Can affect multiple parameters on urine dipstick tests that rely on color changes for interpretation

Specific Parameters Affected:

1. Leukocyte Esterase and Nitrite Tests:

   • These tests are crucial for UTI diagnosis with sensitivities of 83% and 53% respectively 2
   • Phenazopyridine can interfere with the color development in these tests, potentially causing false readings

2. Urinary Ketone Testing:

   • Colorimetric reactions used for ketone detection (nitroprusside method) may be affected 3
   • The nitroprusside reaction produces a purple color that could be masked or altered by the orange-red phenazopyridine pigment

3. Protein and Blood Detection:

   • Tests that rely on color change may show false results due to the intense coloration of phenazopyridine in urine

Clinical Implications

• UTI Diagnosis: The interference with leukocyte esterase and nitrite tests may complicate UTI diagnosis, as these are primary screening tools 2
• Diabetes Monitoring: For patients with diabetes who monitor urinary ketones, results may be unreliable while taking phenazopyridine 3
• Hematuria Assessment: The red-orange color may mask or mimic blood in urine

Recommendations for Clinical Practice

1. Inform Patients:

   • Advise patients that phenazopyridine will cause reddish-orange discoloration of urine 1
   • Warn that it may stain fabric and contact lenses 1

2. Laboratory Testing:

   • When possible, collect urine samples before starting phenazopyridine or after discontinuation
   • If urinalysis is necessary while on phenazopyridine, note medication use on laboratory requisitions
   • Consider alternative diagnostic methods when urinalysis results are critical for clinical decision-making

3. Documentation:

   • Clearly document phenazopyridine use in medical records to prevent misinterpretation of urinalysis results
   • Consider temporary discontinuation of phenazopyridine 24-48 hours before planned urinalysis if clinically appropriate

Additional Considerations

• Duration of Effect: The discoloration and test interference persist while the medication is being taken and may continue for a short period after discontinuation
• Dose-Related Effects: Higher doses may cause more pronounced interference with test results
• Alternative Analgesics: Consider non-phenazopyridine options when accurate urinalysis results are needed

Safety Concerns

While not directly related to urinalysis interference, be aware of rare but serious adverse effects of phenazopyridine:

• Methemoglobinemia, particularly with prolonged use or higher doses 4
• Acute interstitial nephritis 5
• Acute renal failure in susceptible individuals 6
• Sulfhemoglobinemia 7

These potential complications emphasize the importance of using phenazopyridine at appropriate doses and for limited duration.