Which of the following may cause a false‑negative leukocyte esterase result: low urine specific gravity, elevated glucose, exposure of the specimen to air, or absence of white blood cells?

False-Negative Leukocyte Esterase Results

The absence of white blood cells (WBCs) is the primary cause of a false-negative leukocyte esterase result, as the test detects the enzyme produced by WBCs themselves—no WBCs means no enzyme to detect.

Understanding the Leukocyte Esterase Test

The leukocyte esterase test detects an enzyme released by white blood cells in urine as a surrogate marker for pyuria. The test's performance depends fundamentally on the presence of WBCs that produce this enzyme 1.

Causes of False-Negative Results

Absence of White Blood Cells (True Cause)

• This is the actual mechanism for false-negative results, not a "cause" in the traditional sense
• When bacteriuria exists without pyuria (asymptomatic bacteriuria), leukocyte esterase will be negative because there are no WBCs producing the enzyme
• This occurs in approximately 30% of children with positive urine cultures, particularly with non-E. coli organisms 2
• The test distinguishes asymptomatic bacteriuria from true UTI—this is considered an advantage, not a limitation 1

Low Specific Gravity (Documented Interference)

• Low specific gravity does cause false-negative results through dilution effects
• Research demonstrates a statistically significant negative correlation (r = -0.12, p < 0.05) between specific gravity and leukocyte esterase pad performance 3
• This provides partial explanation for the test's limited 58% sensitivity in detecting WBCs in urine sediment 3

Elevated Glucose (Not a Documented Cause)

• The provided evidence does not support glucose as a cause of false-negative leukocyte esterase results
• While glucose can affect other urine parameters, no guideline or research evidence links it to leukocyte esterase interference

Exposure to Air (Timing Issue)

• Specimen freshness is critical: samples must be processed within 1 hour at room temperature or 4 hours if refrigerated to ensure sensitivity and specificity 1
• Delayed processing can affect results, though this relates more to specimen degradation than air exposure per se

Clinical Implications

The leukocyte esterase test has inherent limitations:

• Overall sensitivity: 83% (range 67-94%) 1
• Specificity: 78% (range 64-92%) 1
• Approximately 10% of children with UTIs have negative urinalysis results 2

Critical practice point: In febrile infants and children aged 2 months to 2 years with negative dipstick urinalysis but clinical suspicion for UTI, obtain a urine culture 2. The test cannot replace urine culture for definitive diagnosis 2.

Answer to the Question

Among the options provided:

1. Low specific gravity - YES, causes false-negatives through dilution
2. Elevated glucose - NO documented evidence
3. Exposure to air - Indirectly, through delayed processing
4. Absence of white blood cells - This IS the mechanism of false-negatives in asymptomatic bacteriuria

Low specific gravity is the documented interfering factor that causes false-negative results 3.