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Urine microscopy may improve diagnostic accuracy for urinary tract infection

Tristan Manalac
25 Nov 2018

Relying on urinary symptoms alone may lead to an inaccurate diagnosis of culture-positive urinary tract infection (UTI), according to a recent Singapore study, which also reports that concurrent urine microscopy may improve diagnostic accuracy.

The cross-sectional study included 564 women (age 21 years) who presented with new-onset UTI symptoms 6 months after a prior UTI episode. Mid-stream urine samples were collected and were used for urine microscopy and routine culture procedures. Self-reported or assisted questionnaires were administered to collect demographic and clinical information.

Almost half (45.9 percent; n=259) of the participants were culture-positive for UTI, while 20.0 percent (n=113) showed mixed growth; 34.0 percent (n=192) were culture-negative. Most women with dysuria had culture-positive UTI, while those with foul-smelling urines showed mixed cultures. Almost all participants (96 percent) had two or more symptoms. [Fam Pract 2018;doi:10.1093/fampra/cmy108]

Of the symptoms tested, only dysuria (odds ratio [OR], 1.9; 95 percent CI, 1.2–3.0; p=0.009) and foul-smelling urine (OR, 2.2; 1.4–3.7; p=0.001) were significantly correlated with culture-positive UTI upon multivariate modelling.

Upon inclusion of microscopy results in the model, frequency of micturition (OR, 2.2; 1.1–4.6; p=0.037) and foul-smelling urine (OR, 2.1; 1.1–4.0; p=0.027) emerged as significant factors, while the effect of dysuria was attenuated (OR, 1.3; 0.7–2.4; p=0.429).

Moreover, when urine microscopy reported pyuria, the likelihood of having culture-positive UTI increased from 2.0 to 12.0. Significant visualizations of urine bacteria under microscopy increased the risk of culture-positive UTI (OR, 7.2; p=0.014).

In terms of diagnostic value, combinations of symptoms achieved only a maximum of moderate accuracy for culture-positive UTI. For instance, the combination of dysuria, foul-smelling urine or chills demonstrated a diagnostic accuracy of 61.4 percent. While sensitivity was good (87.6 percent), specificity was poor (26.0 percent).

The second most accurate combination of symptoms was dysuria, haematuria or chills (61.2 percent), which likewise had good sensitivity (83.4 percent) and poor specificity (31.3 percent).

Laboratory results tended to have better diagnostic performance, with urine microscopy reporting pyuria showing good sensitivity (93.7 percent), accuracy (74.4 percent), area under the curve (0.71), and positive (70.9 percent) and negative (85.3 percent) predictive value.

Combinations of symptoms and lab result improved overall diagnostic value. For example, haematuria, dysuria and urgency, along with microscopy-determined pyuria and bacterial count few resulted in a sensitivity of 95.7 percent and accuracy of 74.7 percent. The same was true for the combination of haematuria, incomplete emptying and urgency, with pyuria and bacterial count few (specificity: 95.7 percent; accuracy: 74.7 percent).

“The diagnostic outcomes from the myriad permutations of symptoms and microscopic results can be packaged into an algorithm-based risk calculator. This will ease its application to predict UTI in general practices,” said researchers.

“However, further study is needed to validate the proposed risk calculator and its clinical utility using implementation science,” they noted, adding that there is also a need to evaluate the diagnostic value of the urine dipstix, which is more affordable and more convenient than urine microscopy, but is more accurate than relying on symptoms alone.

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