Biochemical and microscopic examination of the urine is routinely utilized in the evaluation of patients with kidney disorders, and urine microscopy—by providing a view into what's happening in the kidneys—may at times be a surrogate for histologic testing and serve as a “liquid biopsy.”
Several studies have highlighted the benefits of urine microscopy. In patients with a high pretest probability of acute tubular necrosis (ATN), detection of granular casts or renal tubular epithelial cells (RTECs) has a very high positive predictive value and low negative predictive value (1). Application of a urinary sediment score (composite of number of granular casts and RTECs) for patients with ATN serves as a proxy for severity of acute kidney injury (AKI), as it has a dose-dependent relationship with risk of worsening (2). There is also diagnostic utility of serial microscopy in establishing a diagnosis of ATN and other etiologies of a non-recovering or worsening AKI (3).
In order to increase productivity and ensure reproducibility, most laboratories now utilize automated urine flow cytometry or digital imaging systems. Despite these advances in laboratory automation, manual microscopy, particularly one performed by an experienced nephrologist, is usually better at identifying important elements in the urinary sediment, specifically cellular casts, RTECs, and acanthocytes (4, 5). Nevertheless, the trend toward automation has resulted in fewer nephrologists taking the time to look at the urine themselves.
Fortunately, the past few years have witnessed a gradual resurgence of interest in manual microscopy (Figure 1). The expansion of medical education from classroom learning to social media platforms, such as Twitter, has played an essential role. Availability of smartphone microscope adapters has enabled providers to capture high-quality images of the urine sediment for documentation and education. In 2018, the Renal Fellow Network introduced a regular feature, titled “Urine Sediment of the Month,” showcasing images and text contributions from field experts. In 2019, the American Journal of Kidney Diseases featured “Urine sediment examination in the diagnosis and management of kidney disease: Core curriculum,” an extensive review of performance of urine sediment analysis and its role in patient care (6). More recently, NephMadness 2021 introduced the “liquid biopsy region,” highlighting urine microscopy as an important aspect of clinical nephrology.
The incorporation of manual microscopy into dayto-day practice requires proper equipment, experience using different microscopy techniques, and Provider-Performed Microscopy (PPM) certification (https://www.cdc.gov/PPMP), which permits physicians to perform manual microscopy in the office or a laboratory as a part of patient evaluation.
Although there is substantial variability in the interpretation of urine sediment findings among nephrologists (7), continuous medical education, including didactics, computer-based learning, online courses, training workshops, and social media, may help improve interob-server reliability of manual microscopy and ensure continued interest in this time-honored technique.
Perazella MA, et al. Diagnostic value of urine microscopy for differential diagnosis of acute kidney injury in hospitalized patients. Clin J Am Soc Nephrol 2008; 3:1615–1619. doi: 10.2215/CJN.02860608
Perazella MA, et al. Urine microscopy is associated with severity and worsening of acute kidney injury in hospitalized patients. Clin J Am Soc Nephrol 2010; 5:402–408. doi: 10.2215/CJN.06960909
Varghese V, et al. Diagnostic utility of serial microscopic examination of the urinary sediment in acute kidney injury. Kidney360 February 2021; 2:182–191. doi: https://doi.org/10.34067/KID.0004022020
Cho J, et al. Comparison of five automated urine sediment analyzers with manual microscopy for accurate identification of urine sediment. Clin Chem Lab Med 2019; 57:1744–1753. doi: 10.1515/cclm-2019-0211
Tsai JJ, et al. Comparison and interpretation of urinalysis performed by a nephrologist versus a hospital-based clinical laboratory. Am J Kidney Dis 2005; 46:820–829. doi: 10.1053/j.ajkd.2005.07.039
Cavanaugh C, et al. Urine sediment examination in the diagnosis and management of kidney disease: Core curriculum 2019. Am J Kidney Dis 2019; 73:258–272. doi: 10.1053/j.ajkd.2018.07.012
Palsson R, et al. Assessment of interobserver reliability of nephrologist examination of urine sediment. JAMA Netw Open 2020; 3:e2013959. doi: 10.1001/jamanetworkopen.2020.13959