Evaluation of a smartphone-based colorimetric method for urinalysis dipstick readings in cats

Abstract

Objectives: The aim of the study was to compare the diagnostic performances of a smartphone-based colorimetric method (SBCM) for urinalysis with a semi-automated point-of-care (POC) analyser using standardised solutions and cat urine.

Methods: Artificial solutions (negative and positive quality controls, and purposely designed artificial urine) and natural urine from 216 cats were used. Two urine reagent strips were simultaneously dipped in each sample. One dipstick was read by the SBCM and the other by the POC analyser at the same time. Results for pH, proteins, bilirubin, 'blood', glucose and ketones were considered. Overall agreement and sensitivity, specificity and accuracy of the SBCM were determined based on selected cut-offs.

Results: For the artificial solutions, 80 comparisons were obtained for each analyte and each expected concentration. The overall agreement (exactly the same result) between the two methods was 78.4%. SBCM sensitivity, specificity and accuracy were 99.0%, 100% and 99.3%, respectively. The correlation between the two methods was almost perfect (Cohen's kappa coefficient = 0.9851). For natural urine samples, the overall agreement (including pH) was 68.6%. Using optimal cut-offs for the SBCM determined from the results of analysis of artificial solutions, the sensitivity, specificity and accuracy of the SBCM were 100%, 76.02% and 80.5%, respectively. In this situation, the correlation between the two methods was moderate (Cohen's kappa coefficient = 0.5401). This was mostly due to a high rate of false-positive results for bilirubin (61.1%).

Conclusions and relevance: With proper cut-off use (ie, considering positive or negative results) the SBCM evaluated here has a perfect sensitivity and appropriate diagnostic performances for proteins, 'blood', glucose and ketones. Based on these experimental data, this method appears suitable for dipstick urinalysis but positive results for bilirubin and proteins have to be confirmed.

Keywords: Urinalysis; app; colorimetry; dipstick; smartphone.

Figure 1

Percentages of identical and different clinical interpretations for each analyte from the results obtained with the smartphone-based colorimetric method and the point-of-care analyser from 216 cat urine samples

Figure 2

Bubble chart comparing pH results obtained by the smartphone-based colorimetric method (SBCM) and the point-of-care (POC) analyser in 216 cat urine samples. Circle sizes are proportional to the number of results (reported within or near corresponding circles)