Urinary tract infection (UTI) in paediatric patients is one of the most frequent causes of visits in the health care system, in Primary Care as well as in the emergency room, and it is a frequent cause of hospitalisation. In children, urinary tract infection symptoms are usually unspecific, such as fever without a source, food rejection or failure to thrive. That is why diagnostic confirmation with a urine culture is particularly important. Diagnostic methods must be quick and sensitive enough to facilitate prompt treatment and lower mortality rates, but specific enough to avoid unnecessary complementary tests and therapies.

Currently, we have several techniques for the collection of urine samples from children. When the child has bladder control, the technique usually employed is the collection of urine from the urinary stream. In children who do not have bladder control, the most widely used method is the perineal adhesive bag, even though samples collected with this technique have a high risk of contamination or false positives. Samples collected through bladder catheterisation or suprapubic aspiration have a lower risk of contamination, but these are invasive, potentially risky techniques. Therefore, they are used as confirmatory tests or in emergency situations in which an immediate diagnosis or treatment is required.1,2 The use of sterile compresses, a technique employed in other countries, is uncommon in Spain.

Several technical reports and clinical practice guidelines mention studies that show the limitations of the perineal bag for urine culture collection.1,3–5 However, no review studies have as yet quantified the principle problems associated with this sampling technique: the high risk of contamination and false positives. Our objective is to perform a systematic review to estimate the validity of samples collected with a perineal bag as a basis for clinical practice guidelines.

Out of the 47 full-text articles examined, 21 that complied with the inclusion criteria were selected.7–27 The purpose of most of the studies included was to determine the validity of the perineal bag versus other techniques through different estimators. In 15 studies.7,12–14,16–18,21,22,24–27 contaminated samples were estimated, and in 129,15–17,19,20,23,25 false positives were estimated. The validity related to an external reference pattern was determined in only 5 studies.9,11,13,16,17 One article was excluded because it analysed urine samples by a test strip and not by culture.28

Table 1 shows the simplified characteristics of the 21 studies included in the meta-analysis. The studies were carried out in different countries, but their assessment methods and characteristics were very similar. They included a total of 14,759 patients, 7659 with urine bag samples. The ages of the children included in the study are similar, between 1 day and 3 years of age, except for some studies that included some older patients.11,14 The children included had clinical symptoms (most of them with fever without source), suspicion of UTI, or prior positive cultures using a non-sterile urine collection technique. Gender-specific variables are specified in 5 studies,8,11,12,17,23 with a discrete higher frequency of girls.

Table 2 shows the quality assessment of all the studies included in the meta-analysis. The assessment criteria employed for sample contamination or sample positivity/negativity is appropriate, although only one third of the studies use a complete reference pattern for all the samples. In half of the studies, the sample presents some type of bias, mainly in those that assessed sensitivity and specificity, since the analyses do not include contaminated samples. In other studies, the sample includes an over-representation of patients with previously positive urines. None of the studies detail procedures that guarantee the independent or blind assessment of the compared tests, although culture interpretation is a relatively objective test. The biased incorporation of bag samples with positive culture is common in several studies. Finally, all the studies included offer data that enable a correct analysis, although a complete validity assessment can be performed in only one quarter of studies.

Figs. 1 and 2 show the meta-analysis of contaminated samples and false positives with perineal bag. Table 3 shows the pooled measurements for perineal bag and those of other samples analysed in the studies reviewed. For urine collected with the bag, the weighted percentage of contaminated urines is 46.6%, and for false positives 61.1%. In the meta-regression analyses, we found that studies with representative patient samples had a significantly lower percentage of false positives; therefore, we performed a meta-analysis by subgroups. All the analyses show important heterogeneity (I2>50%), although we were unable to identify a particular study that, when excluded, would reduce heterogeneity. The publication bias tests (Begg and Egger) showed no statistical significance.

Figure 1.

Proportion of contaminated samples. 95% CI: 95% confidence interval; Prev: proportion or prevalence.

(0.27MB).

Figure 2.

Proportion of false positives. 95% CI: 95% confidence interval; Prev: proportion or prevalence.

(0.35MB).

In two studies8,17 the data from boys and girls was estimated separately, showing a higher risk of contaminated urine in boys (38.9% vs 29.2%; odds ratio [OR] 1.53; 95% CI: 1.36–1.72). The same was done for false positives in 4 studies, and no significant differences were found (OR: 0.96; 95% CI: 0.25–3.65). For samples collected via urinary stream and bladder catheterisation, the risks of contaminated urine or false positives are clearly lower.

Fig. 3 shows the meta-analysis for validity indicators of culture with perineal bag in comparison with the sterile technique. The weighted specificity and sensitivity are 82% and 88%, respectively. These estimates do not present heterogeneity.

Figure 3.

Sensitivity and specificity.

(0.25MB).

Although the use of urine collection for culture via perineal bag is widespread, the available evidence indicates that it is not a very valid method due to the high risk of contaminated samples and false positives. The studies retrieved from the bibliographic search, however, are extremely heterogeneous; therefore, these findings could be inexact. Although the heterogeneity observed could be unacceptable for relative risk measurements, it is common to find rates of over 90% in meta-analyses of prevalence studies.29–35 In fact, in risk meta-analyses, the baseline prevalence differences between studies, which are partially compensated by the relative measurements, are one of the main heterogeneity determinants.

Even the most optimistic estimates show that this urine collection technique is unreliable. The fact that 1 out of every 2 urine cultures collected with a perineal bag is contaminated or gives a false positive compromises the usefulness of the technique, or should at least make us question the validity of its results.

The high percentages of contamination and false positives are related to the collection procedure, i.e., cleanliness, placement and replacement of the perineal bag. The risk of contamination varies depending on the meticulousness of the procedure followed.10 Based on our results, the risk of urine contamination in children is somewhat higher, although it is unacceptably high in both cases. Although the percentage of false positives is lower in the studies in which the samples do not include an over-representation of patients with prior positive urine cultures, it is, in any case, very high. These studies were not excluded from the meta-analysis because it is common for new samples for culture to be collected from children with prior positive urine cultures (with bag). This tends to overestimate the positive cases in the samples, but it should not affect the confirmation percentage (false positives) of reference techniques.

It is surprising to observe that the 5 studies assessing the validity of sterile collection techniques found them to have a sensitivity and specificity of over 80%. It is evident that the exclusion of contaminated samples (which are neither positive nor negative, and are common in clinical practice) from the validity analysis would bias the estimation. Nevertheless, we cannot rule out the existence of certain settings and patient populations in which these results would be valid if strict hygiene measures are enforced.

The results of this review show the limitations of the studies included. Most of them are of medium quality, since none include such an important criterion as the blind or independent assessment of the diagnostic test and the reference criterion. In addition, the patient samples included are not sufficiently representative due to bias in the inclusion of samples previously positive or contaminated. Likewise, it is surprising that most of the studies are carried out in the hospital environment, although many of the children with UTI symptoms are seen at primary care clinics.

Although different documents and clinical practice guidelines already recommend the use of other urine collection techniques when a reliable diagnosis is essential,1,2 the findings of this review quantify the importance and significance of this recommendation. Considering the high risk of contamination, the perineal bag cannot be used to collect a sample for culture in a high-risk patient requiring immediate antibiotic treatment. A finding of contaminated urine will not confirm or rule out infection, and a positive finding runs a high risk of being false. Also, the antibiotic administered would interfere with the validity of the new urine sample that would have to be collected to confirm diagnosis. The diagnostic uncertainty would affect not only treatment but also the decision to perform further diagnostic tests or follow-up studies. The iatrogenic cascade triggered by each wrong diagnosis must also be considered.

In low-risk patients, in whom diagnosis is unlikely and treatment delay acceptable, we could collect the urine with a bag and wait for the result. The less invasive character and ease of this technique have made it the gold standard in our clinical practice. However, it is important to bear in mind that only negative results will be acceptable because the positive ones will most likely be false and will have to be confirmed with more valid urine collection techniques (bladder catheterisation or suprapubic aspiration). This recommendation, included in consensus documents,1,2 is pragmatic, even assuming that meticulous cleaning, placement and replacement of the bag will minimise the drawbacks of this urine collection technique.

We conclude that the perineal adhesive bag is not a sufficiently valid method for collecting urine for culture, since almost half of the cultures will be contaminated and 2 out of 3 positives will be false. Even though the estimates are inaccurate, due to the heterogeneity of the studies available, they must be considered when choosing the urine collection method. The sensitivity and specificity estimates are not applicable since they do not take into account the risk of contamination.

The authors declare that there are no conflicts of interest.