1. Introduction
Chronic kidney disease (CKD) commonly results from hypertension and diabetes mellitus (DM), with albuminuria serving as an indicator of kidney damage. Timely detection of albuminuria is essential for clinicians to provide early treatment and potentially prevent patients from requiring hemodialysis [
1]. The Kidney Disease: Improving Global Outcomes (KDIGO) 2012 guideline recommends identifying albuminuria using the urine albumin to creatinine ratio (UACR), defining a value of 30 mg/g or more (3 mg/mmol) as the cut-off point [
1]. While the biochemical quantitative method serves as the gold standard for measuring UACR, it is both costlier and more time-consuming compared to reagent strip tests [
2]. Consequently, previous studies have explored reagent strip tests with automatic reading functions as an alternative for diagnosing albuminuria, and they demonstrate promising results [
3,
4,
5].
One of these automated urine reagent strip analyzers is UC-3500 (Sysmex, Kobe, Japan). It employs reflectance photometry with a color complementary metal oxide semiconductor detector (CMOS) to interpret color changes on the reagent strip, resulting in objective results that are less susceptible to human bias. The UC-11A MEDITAPE test strip measures 11 urine parameters, including protein, albumin, and creatinine, and provides semi-quantitative readings for urine protein to creatinine ratio (UPCR) and UACR. The semi-quantitative UACR results (0: <30 mg/g, 1: 30–300 mg/g, 2: >300 mg/g) correspond to the albuminuria categories defined by the KDIGO guideline (A1: <30 mg/g, A2: 30–300 mg/g, A3: >300 mg/g), facilitating the evaluation of albuminuria severity [
1]. Previous studies have reported good accuracy of UC-3500 compared to the biochemical quantitative method [
6,
7].
UC-3500, as outlined in its user manual, utilizes the protein error of pH indicator method to detect urine protein and albumin [
8]. Both pads contain tetrabromophenol blue as the dye for the reaction. However, this dye does not react specifically to urine albumin and can generate false positive results in the presence of detergent, hematuria, immunoglobulin, or other urine proteins. It is important to note that approximately 63–80% of urinary tract infections (UTI) are associated with proteinuria, and 68–91.7% are accompanied by hematuria [
9,
10,
11]. Therefore, the protein and albumin pads may produce false positive results in patients with UTIs.
While previous research has consistently demonstrated that UTIs can lead to a transient increase in urinary albumin, there has been limited exploration of how UTIs may impact the accuracy of reagent strip tests [
12,
13,
14]. The dye used in UC-3500 to detect albumin also reacts with other substances in urine, potentially making it more susceptible to the influence of UTIs. Although this test is convenient for screening albuminuria, result interpretation requires caution. This study aims to investigate whether UTIs affect the accuracy of UC-3500, a reagent strip test, in diagnosing albuminuria by comparing the results with the gold standard method for diagnosing albuminuria, the biochemical quantitative method.
4. Discussion
According to the findings of this study, UTI can influence the diagnosis of albuminuria in two significant ways: (1) the emergence of transient albuminuria that resolves after antibiotic treatment and (2) the occurrence of false positive results in reagent strip tests, such as the UC-3500. UACR serves as a crucial marker for kidney damage in conditions like chronic kidney disease, DM, and hypertension [
1,
20,
21]. As a result, numerous studies have explored the potential use of reagent strip tests as an alternative diagnostic tool for early detection [
3,
4,
5]. However, despite the promising prospects of reagent strip tests, no study, to our knowledge, has ventured into examining the possible impact of UTI on albuminuria diagnosis. Remarkably, even the UC-3500 user manual does not caution that UTI may lead to false positive results on the albumin pad. This study stands as the pioneering investigation to unveil that UTI not only leads to transient albuminuria, but also casts a shadow of doubt on the accuracy of reagent strip tests for albuminuria diagnosis. These findings underscore the necessity for vigilance when employing this convenient screening method for albuminuria, as it is evidently more susceptible to UTI’s influence compared to the biochemical quantitative method. Therefore, the presence of UTI must be rigorously excluded before embarking on the screening process.
Previous studies examining the relationship between UTI and albuminuria have consistently pointed towards a robust association between albuminuria and symptomatic UTI. Additionally, following antibiotic treatment, albuminuria generally shows improvement. In contrast, asymptomatic bacteriuria appears unrelated to albuminuria [
13]. In the present study, we specifically enrolled patients with a UTISA questionnaire score > 3, designating them as individuals with symptomatic UTI. Consequently, our results align with this pattern, revealing a significant reduction in quantitative UACR following antibiotic treatment. Moreover, out of the 35 patients who exhibited albuminuria prior to treatment, 24 (68.5%) witnessed a return to normal urinary albumin levels after treatment, thus conforming to the definition of transient albuminuria.
This substantial reduction was not confined solely to the transient albuminuria group but extended to the persistent and negative albuminuria subgroups (
Figure 1). This implies that UTI can indeed augment urinary albumin excretion in all patients, albeit to varying degrees. Certain patients may even reach the threshold of albuminuria as defined by quantitative UACR. Significantly, the transient albuminuria group exhibited higher levels of leukocyte esterase, WBC, and RBC when compared to the negative albuminuria group (
Table 2). It is plausible that these patients may have endured more pronounced inflammation in the urinary bladder, which could have led to concurrent hematuria or the release of exudate from blood vessels, thereby increasing the amount of albumin in the urine [
22,
23]. In contrast, the persistent albuminuria group showed no significant differences in WBC and RBC compared to the negative albuminuria group. This indicates that the presence of albumin in the urine in this group is not solely due to infection-related inflammation but may be indicative of an underlying condition of albuminuria. Among this group, three patients had known chronic conditions associated with albuminuria, while the remaining eight patients may require further investigation to identify the underlying causes of their albuminuria.
Despite the good correlation between the semi-quantitative results of UACR obtained from UC-3500 and the biochemical quantitative UACR, the agreement between UC-3500 and the biochemical quantitative method was only moderate (κ = 0.49) for diagnosing albuminuria in UTI patients. Nevertheless, this agreement improved to a good level after antibiotic treatment (κ = 0.65). Prior to treatment, the sensitivity and specificity were measured at 88.57% and 57.89%, respectively (
Table 3). Following treatment, the sensitivity and specificity rose to 100.00% and 80.95%, aligning with the results of a previous study that assessed the performance of UC-3500 in a non-UTI population [
2]. The AUC of UC-3500 for diagnosing albuminuria also increased after antibiotic treatment (
Figure 2 and
Figure 3). The augmentation in agreement for classifying albuminuria severity between the two methods was also evident after infection control. These findings underscore the importance of effectively treating the infection to secure reliable results when employing UC-3500 for albuminuria classification.
Furthermore, we observed a heightened false positive rate of UC-3500 for albuminuria diagnosis during UTI. This inaccuracy may stem from the varying specificity to albumin of the dyes used by UC-3500 and the biochemical quantitative method. Bromocresol purple used in the biochemical quantitative method reacts specifically with albumin [
24,
25], in contrast to tetrabromophenol blue used in UC-3500 [
26]. Urine RBC and bacteria concentration were significantly higher in the false positive group (
Table 7), implying their interference with the albumin pad. Hematuria is already acknowledged to cause a false positive reaction [
8], while a high bacteria concentration may elicit a stronger immune response and higher production of immunoglobulin. Nonetheless, the insignificant difference in proteinuria between the false positive and true negative groups contrasts with our expectation. It can be explained by the fact that albumin comprises the primary content of proteinuria during UTI. The comparable incidence of albuminuria (64.82%) in this study to the incidence of proteinuria found in early studies further supports this explanation [
9,
10,
11].
Patients with DM require regular monitoring for the presence of albuminuria or to track changes in its severity. However, DM patients are more susceptible to UTI than the general population [
27]. Consequently, using a reagent strip test for screening or monitoring in this patient group may render the results more susceptible to the influence of UTI. Nevertheless, the test strip of UC-3500 can simultaneously measure leukocyte esterase and nitrite. Furthermore, it can be used in conjunction with an automatic urine particle analyzer to determine the presence of UTI [
28]. This additional information in the test results of these patients can help us assess whether the UACR measured by UC-3500 might be affected by the concomitant presence of UTI. This is a critical factor that makes UC-3500 a more suitable tool for albuminuria screening than the biochemical quantitative method.
This study is not without limitations. Firstly, it constitutes a secondary analysis of data from our prior research [
29], which means we did not assess the within-run and between-run variations of UACR measurement. Secondly, the exclusion of numerous patients due to the UC-3500’s inability to analyze UACR in diluted urine resulted in a relatively limited sample size. This also underscores the challenges in employing UC-3500 for diagnosing albuminuria during UTI, as most UTI patients tend to increase their fluid intake, which can lead to urine dilution. Finally, we utilized post-treatment examination results as the control group to evaluate whether the accuracy of albuminuria diagnosis was indeed affected by the presence of infection. Opting for individuals without UTIs as the control group might have been more appropriate. However, the retrospective nature of this study precluded the use of patients without UTIs as a control group. Nonetheless, the significant improvement in post-treatment UTISA questionnaire scores and the insignificant difference of post-treatment UTISA questionnaire score between the three albuminuria groups suggest that patients with persistent albuminuria may indeed have genuine albuminuria rather than untreated infections. A future study, featuring a larger participant pool and a prospective design, is warranted to further validate the results obtained in this study.