A Urinary Tract Infection (UTI) is a common condition caused when microbes, usually bacteria, enter the urinary tract and multiply. When a patient presents with symptoms such as painful or frequent urination, a medical professional often orders a urinalysis to confirm the infection. While modern rapid tests exist, microscopic examination of the urine remains a reliable standard for viewing the components that indicate a disease process. This procedure provides visual evidence of the body’s immune response and the presence of invading organisms.
The Urinalysis Process
A complete urinalysis begins with a visual examination of the sample, noting its color and clarity; cloudy urine may suggest the presence of white blood cells or bacteria. This is followed by a chemical analysis using a dipstick, which tests for substances like nitrites and leukocyte esterase. These chemical markers correlate with bacterial activity and the presence of immune cells.
The most detailed step is the microscopic examination, which requires concentrating the elements. A sample of fresh urine (10 to 15 milliliters) is placed into a centrifuge, which spins the sample at high speeds for several minutes. This process forces solid components, such as cells, crystals, and bacteria, to collect at the bottom of the tube, forming a pellet called the sediment. The excess liquid (supernatant) is poured off, and the remaining sediment is mixed with the small amount of liquid left behind. A single drop of this concentrated sediment is then placed onto a slide, covered with a coverslip, and prepared for viewing.
Signs of Infection: Bacteria and White Blood Cells
The two most telling signs of an active UTI are a high number of white blood cells (WBCs) and the visible presence of bacteria. The immune response causes an influx of WBCs, primarily neutrophils, into the urinary tract, a condition called pyuria. Technicians quantify these cells by counting them per high-power field (HPF); five or more WBCs per HPF is often considered a positive sign of infection.
The presence of bacteria, known as bacteriuria, is also noted. These microorganisms often appear as small, rod-shaped (bacilli) or spherical (cocci) structures. Rod-shaped bacteria, such as E. coli, are the most frequent cause of UTIs. The amount of bacteria is estimated based on the density seen on the slide, correlating to the “significant bacteriuria” threshold (usually \(10^5\) colony-forming units per milliliter). Observing both abundant bacteria and pyuria provides strong microscopic evidence supporting a UTI diagnosis.
Other Elements and Sample Contamination
While WBCs and bacteria are the primary indicators, other elements are scrutinized, including red blood cells (RBCs) and epithelial cells. The presence of RBCs, or hematuria, is common in UTIs due to inflammation of the urinary tract lining. However, finding more than two RBCs per HPF can also be caused by conditions other than infection, such as kidney stones or trauma. Therefore, hematuria alone is not definitive for a UTI diagnosis.
The quality of the sample is assessed by the number of epithelial cells present, which are shed from the urinary tract lining and external skin. A significant number of squamous epithelial cells (15 to 20 or more per HPF) usually indicates the sample was contaminated during collection from the surrounding skin. This contamination can introduce external bacteria, potentially leading to misleading results and necessitating a repeat test. Crystals, which are typically non-pathogenic formations of mineral salts, may also be seen.
From Slide to Diagnosis: Culture and Sensitivity
The information gathered from the microscopic examination is often a presumptive diagnosis that requires a follow-up test for confirmation and treatment planning. A urine culture is performed by placing a measured amount of the urine sample onto a nutrient-rich plate to allow any present bacteria to multiply. After an incubation period, typically 24 to 48 hours, the number and type of bacterial colonies are counted and identified, providing a definitive confirmation of the microbe causing the infection.
If the culture grows a significant amount of a single type of bacteria, a sensitivity test is performed to guide therapy. This test exposes the isolated bacteria to various antibiotics to determine which ones effectively kill the organism or stop its growth. The results of this sensitivity testing allow the healthcare provider to select the most appropriate antibiotic, directly linking the initial microscopic observation of infection to a precise treatment plan.