Evaluation of the SediMax automated microscopy sediment analyzer and the Sysmex UF-1000i flow cytometer as screening tools to rule out negative urinary tract infections

Highlights

• Two rapid automated analyzers are evaluated for their ability to screen urine samples.

• It is essential to set adequate cutoff points for each setting.

• Patients' medical history may complement the established cutoff points.

• Patients' clinical condition may complement the established cutoff points.

• Both autoanalyzers are good screening tools to rule out negative urine samples.

Abstract

Urinary tract infections (UTI) are highly prevalent in nosocomial and community settings, and their diagnosis is costly and time-consuming. Screening methods represent an important advance towards the final UTI diagnosis, diminishing inappropriate treatment or clinical complications. Automated analyzers have been developed and commercialized to screen and rule out negative urine samples. The aim of this study was to evaluate two of these automated analyzers (SediMax, an automatic sediment analyzer and UF-1000i a flow cytometer) to predict negative urine cultures. A total of 1934 urine samples were analyzed. A very strong correlation for white blood cells (WBC) (r_s: 0.928) and a strong correlation for bacteria (BAC) (r_s: 0.693) were obtained. We also calculated optimal cut-off points for both autoanalyzers: 18 WBC/μL and 97 BAC/μL for SediMax (sensitivity = 96.25%, specificity = 63.04%, negative predictive value = 97.97%), and 40 WBC/μL and 460 BAC/μL for UF-1000i (sensitivity = 98.13%, specificity = 79.16%, negative predictive value = 99.18%). The use of SediMax and UF-1000i resulted in a 46.33% and 57.19% reduction of all samples cultured, respectively. In conclusion, both analyzers are good UTI screening tools in our setting.

Introduction

Urinary tract infections (UTI) are common in both the hospital and community setting, acquiring high prevalence rates [1], [2]. Clinical symptoms like dysuria, frequent urination, hematuria and back pain are not considered specific enough to reach a UTI diagnosis [3]. Quantitative urine culture is still considered the gold-standard for a UTI diagnosis in clinical laboratories, but this has its limitations.

Due to the large number of samples processed, classic urinalysis based on microscopic sediment examination followed by urine culture is laborious and time-consuming, and many samples yield a negative result (up to 80% of urine cultures) [4]. In addition, manual sediment examination lacks adequate standardization and can lead to misinterpretations. Alternative methods have been developed in an effort to attain a reliable UTI diagnosis. Some are based on chemical dipsticks which detect leukocyte esterase or nitrites. While they are proving very useful in UTI screening [5], [6], they have also been associated with many false positive and negative results [7], [8].

Over the past few years, automated urinalysis systems have gained significant importance in UTI diagnosis. There are several autoanalyzers commercialized, and many have shown good correlation with culture in identifying negative urine samples; for example, IRIS iQ200 (Iris Diagnostics, Chatsworth, CA) [9], [10] or FUS200 (Changchun Dirui Industry, China) [11]. Two of the latest analyzers are SediMax, an automated microscopy urine sediment analyzer (also known as UriSed) (77 Elektronika, Budapest, Hungary) and the Sysmex UF-1000i flow cytometer (TOA Medical Electronics, Kobe, Japan). They have been evaluated separately over the past few years, with varying results depending on the population [12], [13], [14], [15], [16], [17], [18]. Another alternative screening method to obtain faster results is the automated bacterial culture device, Alfred 60/AST (Alifax, Padua, Italy), which uses laser nephelometry to detect and quantify bacterial growth. A study by Lahans et al. in Australia showed many false negative results, so this should be combined with microscopy [19].

Effective screening in the clinical laboratory would reduce the laboratory workload, improve turnaround times and throughput, as well as reducing the costs of UTI diagnosis. High sensitivities and negative predictive values are primordial to avoid classifying positive urine samples as negative. The rapid screening of a suspected UTI is important to help reduce the inappropriate use of antibiotics and, consequently, avoid the spread of multi-drug-resistant bacteria, which are associated with poorer clinical and economic outcomes [20], [21].

The aims of this study were (i) to compare the performance of two autoanalyzers (SediMax automated microscopy urine sediment analyzer and Sysmex UF-1000i flow cytometer) in terms of urine particle detection, especially white blood cells, red blood cells, epithelial cells, bacteria and yeast; and (ii) to evaluate their capacity to predict negative urine samples, as well how well these might adapt to a high-volume laboratory.