Study objective: Frequently, attempts to obtain catheter urine samples from infants are unsuccessful because of an empty bladder, with urinary catheterization failure rates reported up to 16%. We seek to validate a sonographic urinary bladder index that will identify patients for whom catheterization will be unsuccessful.
Methods: We conducted a prospective, blinded, observational study enrolling a convenience sample of children younger than 2 years and undergoing urinary catheterization in an urban pediatric emergency department. Failure was defined as a catheterization result of less than 2 mL of urine. Urinary bladder index, a concept we created, is defined as the product of anteroposterior and transverse diameters, expressed in centimeters squared. Sensitivity and specificity were calculated with 95% confidence intervals (CIs).
Results: Forty-four patients were enrolled, with a median age of 8 months (range 0.5 to 24 months) and median weight of 7.8 kg (range 1.7 to 17.7 kg). Four children urinated during the procedure and were excluded. Thirty-five had urinary bladder index greater than 2.4 cm2 (range 2.5 to 15.5 cm2 ). All were successfully catheterized. Four children had urinary bladder index less than 2.4 cm2 (range 0 to 1.2 cm 2 ). No adequate samples were obtained from those children. The bladder was not visualized at all in 1 child who was successfully catheterized. Sensitivity of the urinary bladder index to identify failure to obtain 2 mL of urine was 100% (4 of 4; 95% CI 40% to 100%), specificity was 97% (35 of 36; 95% CI 85% to 100%).
Conclusion: A urinary bladder index less than 2.4 cm2 appears to identify infants for whom urinary catheterization will be unsuccessful because of insufficient urine volume. Ultrasonographic evaluation with urinary bladder index measurement before catheterization may eliminate the incidence of failed procedures.