The quality of evidence on bladder volume estimation is downgraded by inconsistency (unexplained variability of results). The quality of evidence on the reduction of urinary tract infections is upgraded by large effect size.
Ultrasonography is recommended over bladder catheterization for bladder volume estimation
The recommendation attaches a relatively high value on avoiding adverse effects (urinary tract infection) and inconvenience of catheterization.
A systematic review 1 including 16 studies was abstracted in DARE. 3 studies (n=81) focused on both inter-observer reliability and validity, while 13 studies (n=504) assessed only validity of ultrasonic estimation of bladder volume. There were 21 different formulae for calculating bladder volume reported in these studies. The most frequently used formula: height x width x depth x correcting factor was reported in 5 studies. The correcting factor ranged from 0.625 to 1.00. In the tree studies assessing reliability, the indices for reliability were 0.923, 0.99 and 1.00. In the validity studies, the indices of concordance with reference measure ranged from 0.914 to 0.983.
The precision of five different calculation methods in real-time bladder volume measurement was assessed in 10 volunteers, with bladder volumes ranging from 120 ml to 465 ml 2. Five different calculation algorithms were used on the frozen ultrasound pictures: the prolate ellipsoid method based on the formula: volume = length x width x heigth x 0.52 on two dimensions; the double area method based on the formula: volume = ex [C1 + C2 x ln(A1) + C3 x ln(A2)] on two dimensions; The double ellipsoid method on two dimensions; the method of one dimension of the shape of the bladder outlined manually with the maximal longitudinal diameter; the method of one dimension of the shape of the bladder outlined by smooth ellipsoid with the maximal longitudinal diameter After registration of the voided volume the subjects were scanned again in order to assure complete emptying.The precision of each method was equal. Errors were not more than 25% of the voided volume.
Three observational studies were included in a meta-analysis on the effect of ultrasonography versus bladder catheterization on urinary infection incidence in hospitalized patients 3. The overall effectiveness of the bladder ultrasound scanner in the reduction of urinary tract infection associated with catheterisation was OR 0.27 (IC95% 0.16-0.47; p-value 0.00000294, variance 0.08, weight 12.50).
Comment: The quality of evidence is downgraded by inconsistency of results. The study designs and the potential elements of bias make it impossible to recommend the most valid formulae for estimating bladder volume. The variation in bladder shape at different volumes would suggest that a single formula may be inappropriate at different bladder volumes.
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