Can Azo (phenazopyridine) cause false positive nitrate results in urinalysis?

Phenazopyridine (Azo) Does Not Cause False Positive Nitrates in Urinalysis

Phenazopyridine (Azo) does not cause false positive nitrate results in urinalysis. While phenazopyridine can cause other urinalysis abnormalities and urine discoloration, there is no evidence in the medical literature that it affects the nitrite test specifically 1, 2.

Understanding the Nitrite Test in Urinalysis

• The nitrite test detects the presence of nitrites in urine, which are formed when certain bacteria convert urinary nitrates to nitrites through metabolism 3, 1
• The nitrite test has high specificity (98%) but lower sensitivity (53%) for detecting urinary tract infections 1, 2
• A positive nitrite test strongly suggests bacterial infection, particularly with gram-negative enteric bacteria 3, 1
• False negative nitrite results are common due to:
  • Frequent voiding (insufficient time for nitrate conversion) 3, 2
  • Infections with non-nitrite-producing organisms like Staphylococcus saprophyticus 2
  • Low dietary nitrate intake 4
  • Dilute urine 4

Known Effects of Phenazopyridine (Azo) on Urine

• Phenazopyridine is a urinary analgesic that causes orange discoloration of urine 5, 6
• Potential side effects include methemoglobinemia, yellowish skin discoloration, hepatitis, and in rare cases, acute renal failure 5, 6
• While phenazopyridine can affect urine color, there is no documented evidence that it interferes with the nitrite test in urinalysis 1, 2

Interpreting Urinalysis Results

• For optimal UTI diagnosis, combine multiple parameters: leukocyte esterase, nitrite test, and microscopy (99.8% sensitivity) 1, 2
• Leukocyte esterase has better sensitivity (83%) but lower specificity (78%) compared to nitrites 1
• The combination of leukocyte esterase OR nitrite positive increases sensitivity to 93%, making it a better screening tool 1
• False positive urinalysis results are common even with ideal collection techniques 7
• Among urinalysis parameters, nitrite showed the highest specificity in disease-free populations 7

Clinical Implications

• When evaluating a patient on phenazopyridine with suspected UTI, the nitrite test remains reliable 1, 2
• A positive nitrite test should still be interpreted as suggestive of bacterial infection 3, 1
• Remember that a negative nitrite test does not rule out UTI due to its low sensitivity 1, 2
• For definitive diagnosis, urine culture remains the gold standard 1, 4

Potential Pitfalls in UTI Diagnosis

• Do not rule out UTI based solely on negative nitrite results 1, 2
• Do not diagnose UTI based on positive culture alone without evidence of pyuria 1
• Avoid collecting specimens in urine bags for definitive diagnosis; while negative results from bagged specimens may be useful, positive results require confirmation with catheterization or suprapubic aspiration 3, 1
• Remember that urine specimens must be processed promptly (within 1 hour at room temperature or 4 hours if refrigerated) to ensure accurate results 1