8.4 Cerebrospinal Fluid Analysis, Cell Counts & Cytology

Cerebrospinal Fluid Analysis, Cell Counts & Cytology

The ASCP BOC includes body fluids inside BOTH the physiology content area and the laboratory testing content area, and the composite reference range table supplies CSF values (WBC and RBC 0 to 5 per microlitre) that examinees are expected to apply. Cerebrospinal fluid is the highest-stakes fluid the hematology bench handles: cell counts are performed manually on a very small, non-renewable volume, and the differential is prepared by cytocentrifuge.

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Cerebrospinal Fluid (CSF) Analysis & Microscopic Cytology

Cerebrospinal fluid (CSF) is collected via lumbar puncture between the L3–L4 or L4–L5 intervertebral spaces. The standard diagnostic protocol utilizes a four-tube collection sequence to avoid pre-analytical contamination:

• Tube 1: Chemistry and Immunology (Protein, Glucose, Electrophoresis, Oligoclonal bands).
• Tube 2: Microbiology (Gram stain, Bacterial and Fungal cultures, Molecular PCR panels).
• Tube 3: Hematology (Total cell count and cytospin differential count).
• Tube 4: Specialized testing (Cytopathology, Flow cytometry, or reference infectious disease testing).

Adult CSF Reference Values vs. Neonatal Values

• Adults: Total WBC count: $0\text{ to }5/\mu\text{L}$ (differential: $70% \pm 15%$ lymphocytes, $30% \pm 15%$ monocytes/macrophages, $0%$ neutrophils); RBC count: $0/\mu\text{L}$; Total Protein: $15\text{ to }45\text{ mg/dL}$; Glucose: $45\text{ to }80\text{ mg/dL}$ (representing $60%\text{ to }70%$ of simultaneous plasma glucose).
• Neonates (0–28 days): Immature blood-brain barrier permits normal WBC counts up to $0\text{ to }30/\mu\text{L}$ (predominantly monocytes and lymphocytes) and protein levels up to $150\text{ mg/dL}$.

Traumatic Tap versus Subarachnoid Hemorrhage (SAH)

                  CSF HEMORRHAGE DIFFERENTIATION PROTOCOL
                                     │
                       [ Grossly Bloody CSF Sample ]
                                     │
            ┌────────────────────────┴────────────────────────┐
            ▼                                                 ▼
    [ Traumatic Lumbar Puncture ]                  [ Subarachnoid Hemorrhage ]
  - RBC count decreases markedly                 - RBC count remains uniform
    from Tube 1 ──> Tube 4                         across Tubes 1 through 4
  - Supernatant: Crystal Clear                   - Supernatant: Xanthochromic
    post-centrifugation                            (pink/yellow within 2–12 hours)
  - Clotting: May form fibrin clot               - Clotting: Non-clotting
  - Cytology: Intact fresh RBCs;                 - Cytology: Erythrophages &
    no hemosiderin/siderophages                    hemosiderin-laden siderophages

Meningitis CSF Differential Patterns

• Acute Bacterial Meningitis: Markedly elevated WBC count ($>1,000\text{ to }10,000+/\mu\text{L}$) with $>80%$ neutrophils; marked protein elevation ($>100\text{ to }500+\text{ mg/dL}$); markedly decreased glucose ($<40\text{ mg/dL}$ or $<40%$ of simultaneous plasma glucose); positive Gram stain/culture.
• Viral (Aseptic) Meningitis: Moderately elevated WBC count ($50\text{ to }500/\mu\text{L}$) with lymphocytic pleocytosis ($>70%\text{--}90%$ lymphocytes/reactive lymphocytes); normal glucose ($>60%$ plasma); normal to mildly elevated protein ($50\text{ to }100\text{ mg/dL}$).
• Fungal / Tubercular Meningitis: Moderate pleocytosis ($100\text{ to }500/\mu\text{L}$) with mixed lymphocytes and monocytes; markedly elevated protein ($>100\text{ to }500\text{ mg/dL}$); decreased glucose; positive India ink preparation or Cryptococcal Antigen (CrAg) lateral flow assay.

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Manual CSF Cell Counting and the Dilution Calculation

CSF cell counts are performed on the undiluted fluid whenever the specimen is clear, using the Improved Neubauer hemocytometer. The general formula is the same one used for blood:

$\text{Cells}/\mu\text{L} = \frac{\text{Cells counted} \times \text{Dilution factor} \times 10}{\text{Area counted (mm}^2)}$

• Undiluted clear fluid: count all nine large squares on both sides of the chamber. With an area of 18 mm$^2$ across both sides and a dilution factor of 1, the count is (cells x 10) / 18.
• Turbid or bloody fluid: dilute with saline (for total cell counts) or with 3% glacial acetic acid (to lyse red cells for a leukocyte-only count), and multiply by the dilution factor.
• Worked example. A clear CSF is counted undiluted; 36 leukocytes are seen across all nine squares of one side (area 9 mm$^2$). Cells/uL = (36 x 1 x 10) / 9 = 40/uL, a pleocytosis well above the 0 to 5/uL reference interval.

Correcting a Traumatic Tap

When blood contaminates the specimen, the leukocytes contributed by that blood must be subtracted before a true pleocytosis is reported. The simplest approach uses the patient's own peripheral blood ratio:

$\text{WBC added} = \text{CSF RBC} \times \frac{\text{Blood WBC}}{\text{Blood RBC}}$

Subtract that value from the measured CSF WBC. Timing matters as well: cells in CSF deteriorate rapidly, so counts and cytospin preparations must be completed within 1 hour of collection, and the specimen is never refrigerated when culture is also requested.