A urinary tract infection (UTI) occurs when bacteria enter and multiply within the urinary system. Diagnosing a UTI often involves a microscopic examination of a urine sample, which helps identify signs of infection.
Collecting and Preparing the Sample
Collecting a urine sample for microscopic analysis begins with a “clean catch” midstream technique. This method aims to minimize contamination from bacteria and cells present on the skin around the urinary opening. For this, individuals are typically instructed to clean the genital area with antiseptic wipes and then begin urinating into the toilet before collecting the middle portion of the urine stream into a sterile container.
Once collected, the sample is transported to a laboratory for preparation. Laboratory technicians centrifuge the urine sample, which involves spinning it at high speeds. This process concentrates any solid components, such as cells, bacteria, and crystals, into a small pellet at the bottom of the tube. The liquid portion, known as the supernatant, is then discarded, leaving behind the concentrated sediment for microscopic viewing.
Key Elements Under the Lens
Under a microscope, several elements can indicate the presence of a UTI. The primary indicators are bacteria and white blood cells, but other components like red blood cells, yeast, and epithelial cells are also observed.
Bacteria
Bacteria are single-celled organisms that appear as tiny, often colorless shapes. They can be seen as spheres (cocci), rods (bacilli), or sometimes spirals (spirilla). While a small number of bacteria can be normal due to external contamination, their presence in significant numbers, especially when accompanied by other signs of infection, strongly suggests a UTI.
White blood cells (WBCs)
White blood cells (WBCs) are part of the body’s immune response to infection or inflammation. Under the microscope, they appear as round cells, larger than red blood cells, and may show granules within them. An increased number of white blood cells in urine indicates inflammation, which is often a response to a bacterial infection in the urinary tract.
Red blood cells (RBCs)
Red blood cells (RBCs) are typically round, biconcave discs, appearing yellowish or colorless under the microscope. Their presence in urine is called hematuria. While a few red blood cells might be normal, higher numbers can indicate irritation or inflammation of the urinary tract lining, which can be associated with UTIs.
Yeast cells
Yeast cells, particularly Candida albicans, can also cause UTIs, especially in individuals with weakened immune systems or diabetes. These appear as oval or round structures, often showing “budding” (smaller, newly formed cells attached to the parent cell) or forming longer chains called pseudohyphae. Their presence suggests a fungal infection rather than a bacterial one.
Epithelial cells
Epithelial cells are cells shed from the lining of the urinary tract. There are different types, including squamous, transitional, and renal tubular epithelial cells. A small number of squamous epithelial cells are a normal finding, often indicating some contamination from the external genital area during sample collection. However, a large quantity of epithelial cells, or the presence of transitional or renal epithelial cells, can suggest inflammation, irritation, or damage within the urinary tract.
Interpreting the Microscopic Findings
The presence of bacteria, especially in conjunction with an elevated white blood cell count, is a strong indicator of a UTI. For bacteria, a count often exceeding 100,000 per milliliter suggests an active infection. However, some guidelines may consider lower thresholds, particularly in symptomatic patients or specific populations.
Similarly, an increased number of white blood cells, typically more than 5-10 white blood cells, points to inflammation. This condition, known as pyuria, often accompanies bacterial UTIs, as the body dispatches immune cells to combat the infection. While pyuria strongly suggests inflammation, it does not always confirm a bacterial infection, as other conditions can also cause its presence.
Microscopic findings are generally considered alongside a patient’s clinical symptoms, such as painful urination, frequent urination, or abdominal pain. If the microscopic results indicate infection, a urine culture and sensitivity test is often performed. This test identifies the specific type of bacteria or yeast causing the infection and determines which antibiotics will be most effective in treating it.