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Urinalysis & Body Fluids

Microscopic Analysis

We have classified our serous fluid as a transudate or an exudate using chemical markers. For transudates, the story often ends there. But for an exudate, the most important chapters are yet to be written. The microscopic analysis allows us to read that story directly

By examining the cells within the fluid, we move beyond indirect markers of inflammation like protein and LDH. We can now answer specific, crucial questions:

  • What is the magnitude of the cellular response? (Total Cell Count)
  • What is the nature of the response - is it acute bacterial, chronic, allergic, or malignant? (The Differential)
  • Is the cause of the problem visible? (Bacteria, Malignant Cells)

This analysis, performed on an EDTA-anticoagulated tube to preserve cellular integrity, provides a visual diagnosis that complements and often surpasses the chemical findings

Total Cell Counts

This is our first quantitative look at the cellularity of the fluid

Methodology

  • WBC Count: Performed using a hemacytometer, either manually or on an automated cell counter (if validated for body fluids). If the sample is bloody, RBCs must be lysed first using a hypotonic saline or weak acid solution
  • RBC Count: Performed on the undiluted fluid if not grossly bloody

Interpretation

  1. WBC Count
    • Transudates: Typically have a low WBC count, < 1000/µL
    • Exudates: Typically have a WBC count > 1000/µL
    • Clinical Thresholds
      • Pleural Fluid > 10,000/µL: Suggests a parapneumonic effusion or empyema
      • Peritoneal Fluid > 250/µL (with >50% Neutrophils): This is a critical threshold, highly suggestive of Spontaneous Bacterial Peritonitis (SBP) in a patient with cirrhosis
  2. RBC Count
    • Normal/Transudate: Very few RBCs present
    • > 100,000/µL: Indicates a hemorrhagic effusion. This finding significantly increases the suspicion for malignancy, trauma, or pulmonary infarction. A traumatic tap must always be considered, but a consistently bloody fluid points toward true pathology

WBC Differential - The Heart of the Analysis

This is the most diagnostically powerful part of the microscopic exam. It tells us which type of immune cell is responding to the insult. As with CSF, we must use a cytocentrifuge (Cytospin) to concentrate the cells for a Wright-Giemsa stained slide

Cellular Cast of Characters

(To do: add cytospin images of each cell type)

  1. Neutrophils
    • Appearance: Multi-lobed nucleus, granular cytoplasm
    • Significance: The hallmark of acute inflammation and bacterial infection
    • Clinical Correlation
      • >50% Neutrophils: in pleural fluid strongly suggests a parapneumonic effusion
      • >50% Neutrophils: with a peritoneal WBC count >250/µL is the diagnostic criteria for SBP
      • Also elevated in pancreatitis and early tuberculosis
  2. Lymphocytes
    • Appearance: Round nucleus, scant cytoplasm
    • Significance: The hallmark of chronic inflammation
    • Clinical Correlation
      • Tuberculosis: A classic cause of a lymphocytic-predominant exudate
      • Malignancy: Both lymphomas and the host’s immune response to a carcinoma can cause a lymphocytic effusion
      • Rheumatoid Arthritis & Sarcoidosis: Autoimmune conditions leading to chronic inflammation
  3. Macrophages (Monocytes)
    • Appearance: Large cells with an indented or folded nucleus and abundant, often vacuolated cytoplasm
    • Significance: The “clean-up crew.” Seen in all types of effusions but predominate in more chronic conditions alongside lymphocytes. Their morphology can provide clues (e.g., hemosiderin-laden in old hemorrhage)
  4. Mesothelial Cells
    • Appearance: The most challenging cell. These are the cells that line the serous membranes. They can be shed into the fluid and become “reactive” in response to any inflammation
    • Reactive Features: They can appear large, have prominent nucleoli, form clusters, and have a high N:C ratio, making them difficult to distinguish from malignant cells.
    • Key Differentiating Features: They often have a “window” or clear space between cells in a cluster, a “fried egg” appearance (central nucleus, round cytoplasm), and a smoother nuclear contour compared to cancer cells
    • CRITICAL POINT: The absence of mesothelial cells in a pleural fluid exudate is a suspicious finding, as extensive tumor growth or fibrosis (like in TB) can “pave over” the pleural surface, preventing them from being shed
  5. Eosinophils (>10%)
    • Significance: Defines an eosinophilic effusion
    • Clinical Correlation: Most commonly associated with the presence of air or blood in the pleural space (pneumothorax, hemothorax). Also seen in parasitic infections, allergic reactions, and some malignancies. It’s often a non-specific finding
  6. Malignant Cells
    • The Ultimate Target.: The definitive diagnosis of a malignant effusion rests on identifying these cells
    • Appearance (The hallmarks of malignancy)
      • Cellular Clustering: Tend to form tight, three-dimensional balls or morules
      • High N:C Ratio: Very large nucleus with little cytoplasm
      • Irregular Nuclear Contour: Jagged, notched, or convoluted nuclear membranes
      • Irregular Chromatin: Clumped, unevenly distributed chromatin
      • Prominent/Irregular Nucleoli: Large, often multiple nucleoli
      • Cellular Cannibalism: One tumor cell engulfing another
    • Common Primaries: Lung and breast adenocarcinoma are the most common causes of malignant pleural effusions. Ovarian and GI cancers are common causes of malignant ascites

Microbiological Stains

  • Gram Stain: Always performed on exudative fluids, especially when infection is suspected. A positive Gram stain is a critical result. Sensitivity is low, but specificity is high
  • Acid-Fast Stain (AFB): Performed when tuberculous pleuritis is suspected. As with CSF, the sensitivity is very low. Culture and ADA levels are more reliable

Conclusion: Correlating Patterns

Let’s put it all together. You receive a pleural fluid with the following results:

  • Appearance: Turbid, yellow
  • WBC Count: 8,500/µL
  • RBC Count: 20,000/µL
  • Differential: 88% Neutrophils, 10% Lymphocytes, 2% Macrophages
  • Chemistry: Fluid Protein 4.5 g/dL (Serum 6.8), Fluid LDH 500 U/L (Serum 180)
  • Glucose: 35 mg/dL (Serum 100)

Your Diagnostic Synthesis 1. Classification: The fluid protein/serum ratio is 0.66 (>0.5) and the LDH is high. This is clearly an exudate by Light’s Criteria 2. Characterization: The WBC count is high with a marked neutrophilic predominance. This points to an acute bacterial process 3. Confirmation: The glucose is very low, consistent with bacterial consumption 4. Conclusion: This is a classic profile of a parapneumonic effusion or empyema. The next step is a Gram stain and culture

This is the level of synthesis required. You don’t just report the numbers; you see the pattern and understand the disease it represents