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Urinalysis & Body Fluids

Microscopic

Let’s get our slides on the stage. The microscopic exam is arguably the most exciting part of body fluid analysis. This is where we leave the objective numbers of the chemistry analyzer behind and use our own eyes and brains to become cellular detectives. It’s our chance to be the pathologist at the bench, directly observing the disease process. We’re looking for the cellular cast of characters: the heroes (WBCs), the villains (bacteria, malignant cells), and the innocent bystanders (RBCs, tissue cells)

Core Principles of Microscopic Analysis

Before we dive into specific fluids, let’s cover the “how-to.” Getting a good look at the cells in a typically cell-poor fluid requires a few key steps

  • Cell Counting with a Hemacytometer: This is the classic manual method for getting a quantitative cell count (e.g., cells per microliter). We load the fluid into a special glass slide with a grid of a known area and depth. By counting the cells in specific squares, we can calculate the concentration. It’s crucial for fluids like CSF where even a few extra cells are clinically significant
  • Concentrating the Specimen for Differential: To identify the types of cells present, we need to concentrate them. The gold standard is the cytocentrifuge (e.g., Cytospin). This instrument spins the fluid and gently deposits the cellular elements in a dense, monolayer button on a microscope slide. This concentration is essential for finding rare but critical cells, like malignant cells
  • Staining: Once the cells are on the slide, we stain them, typically with a Wright-Giemsa stain, just like in hematology. This allows us to differentiate neutrophils, lymphocytes, macrophages, and other abnormal cells based on their nuclear and cytoplasmic features

Fluid-Specific Microscopic Findings

Cerebrospinal Fluid (CSF)

The microscopic exam of CSF is often a life-or-death situation. Normal CSF is practically acellular

  • Cell Count
    • Normal: 0-5 WBCs/µL for an adult. Anything above this is called pleocytosis and indicates pathology
    • Traumatic Tap vs. Hemorrhage: We can use RBC counts to help. In a traumatic tap, the RBC count will decrease from tube 1 to tube 3. In a subarachnoid hemorrhage, the count will be consistent across all tubes. Furthermore, after centrifugation, you may see erythrophages (macrophages that have ingested RBCs) in a true hemorrhage
  • The Differential: This is where we diagnose meningitis!
    • Neutrophils: A high number of neutrophils (neutrophilic pleocytosis) is the hallmark of bacterial meningitis. You are on the front lines, seeing the body’s acute inflammatory response. Look for intracellular bacteria!
    • Lymphocytes: A high number of lymphocytes (lymphocytic pleocytosis) points towards viral meningitis. It can also be seen in later stages of fungal or tuberculous meningitis
    • Eosinophils: A finding of >10% eosinophils is significant and can suggest a parasitic infection, a fungal infection (especially Coccidioides), or an allergic reaction to a shunt
    • Malignant Cells: Finding clumps of large, abnormal cells with high nuclear-to-cytoplasmic ratios, irregular nuclear membranes, and prominent nucleoli can be diagnostic of carcinomatosis (cancer that has spread to the CNS)

Synovial Fluid (Joint Fluid)

The star of this show isn’t just the cells—it’s the crystals!

  • Cell Count & Differential
    • A high WBC count (>2,000/µL) suggests inflammation. A count >50,000/µL with a predominance of neutrophils (>75%) is a critical finding highly suggestive of septic (bacterial) arthritis
  • Crystal Identification: This is the definitive test for crystal-induced arthropathies. We use compensated polarized light microscopy
    • Monosodium Urate (MSU) Crystals: Diagnostic for Gout
      • Shape: Fine, needle-like crystals
      • Birefringence: Strongly and negatively birefringent.
      • Color ID: When the crystal is aligned parallel to the slow axis of the compensator, it appears yellow. (Mnemonic: parallel = yellow). They are blue when perpendicular
    • Calcium Pyrophosphate Dihydrate (CPPD) Crystals: Diagnostic for Pseudogout
      • Shape: Smaller, rhomboid-shaped or square-like crystals
      • Birefringence: Weakly and positively birefringent.
      • Color ID: When the crystal is aligned parallel to the slow axis of the compensator, it appears blue. (Mnemonic: Parallel = Positive = blue)

Serous Fluids (Pleural, Peritoneal, Pericardial)

Here, the major microscopic goal is differentiating infection from malignancy

  • Cellular Composition
    • Neutrophils: A high percentage points to a bacterial infection (e.g., an empyema in pleural fluid or bacterial peritonitis)
    • Lymphocytes: A predominance can suggest tuberculosis or malignancy (especially lymphoma)
    • Malignant Cells: This is a major focus. We look for the classic signs of malignancy: large cells in 3D clusters or “cannonballs,” high N:C ratio, irregular nuclear chromatin, and cytoplasmic vacuoles
    • Mesothelial Cells: These are the normal lining cells of the serous cavities. They can become very “reactive” in inflammatory states, making them large and sometimes difficult to differentiate from malignant cells. This is one of the biggest challenges in serous fluid cytology

Semen Analysis

This is a comprehensive microscopic evaluation of fertility potential

  • Sperm Count (Concentration)
    • Performed on a hemacytometer. Normal is defined as >20 million sperm/mL
  • Sperm Motility
    • A wet prep is observed to assess the percentage of sperm that are moving and the quality of their movement. We grade motility, for example, on a scale of 0 (immotile) to 4 (rapidly progressive). Good forward progression is key
  • Sperm Morphology
    • A stained smear (like a blood smear) is evaluated. At least 200 sperm are assessed for abnormalities of the head, midpiece, and tail. Strict criteria (like Kruger’s) are used, where even minor defects count as abnormal. A normal sample will have >4% normal forms by these strict criteria
  • WBCs
    • An increased number of WBCs (>1 million/mL) indicates infection or inflammation, which can negatively impact fertility. A peroxidase stain is often used to differentiate WBCs (peroxidase positive) from immature sperm cells/spermatids (peroxidase negative)

Feces

  • White Blood Cells (specifically Neutrophils)
    • The presence of fecal WBCs is a critical finding. It indicates the intestinal wall has been invaded by an infectious organism (Shigella, Salmonella, Campylobacter, E. coli O157:H7) or is inflamed due to a condition like ulcerative colitis. They are generally absent in diarrhea caused by viruses or toxins
  • Fat (Steatorrhea)
    • Staining a sample with Sudan III or Oil Red O: allows us to visualize fat. The presence of a large number of large, orange-red fat globules is indicative of malabsorption
  • Ova & Parasites (O&P)
    • While a specialized area of microbiology, the microscopic search for protozoan trophozoites/cysts and helminth eggs/larvae is a major component of fecal analysis

Putting It All Together

Think of the microscope as the ultimate truth-teller. The chemical results might shout “inflammation!” but the microscopic exam whispers why. It shows you the neutrophils swarming bacteria in a septic joint. It reveals the needle-sharp MSU crystal that is the source of a patient’s agonizing gouty attack. It provides the visual evidence of malignant cells that confirms a devastating diagnosis. Your skill on the microscope doesn’t just generate a result; it paints a picture of the disease itself, providing clinicians with the direct visual evidence they need to make the right call