CBC Reviewer

COMPLETE BLOOD COUNT (CBC) — EXAM REVIEWER

MLS 417-LAB | Hematology

WHAT IS CBC?

• Comprises RBC, WBC, and platelets

• Most commonly ordered laboratory test

• Done via automated analyzer; manual method only for suspicious/critical results

• Provides info about the hematopoietic system and screens other organ systems

2 PHASES OF CBC INTERPRETATION

ASSESSING RESULTS RELATIVE TO REFERENCE INTERVALS

Different reference intervals exist for:

• Men, Women, Children (different ages), Age groups

Notable: WBC is notably higher in children

Units: SI units or common units

Strategies for Determining Significance:

1. Very far from reference interval → likely pathologic → perform manual method

2. Two or more related parameters slightly/moderately outside the interval in the same direction → clinically significant

3. Delta Check — best comparison is the patient's own prior results when healthy (not just the reference interval)

WBC PARAMETERS

WBC-Related Parameters:

Steps in WBC Interpretation:

Step 1 — Ensure accurate WBC count

• Automated instruments can eliminate NRBCs that falsely increase WBC

• Manual: mathematically correct for NRBCs

• Compare WBC histogram/scatterplots with cell counts

Step 2 — Look at the WBC count

• ↑ = Leukocytosis

• ↓ = Leukopenia

Step 3 — Examine relative differential counts

• Reported in percentage, compared to reference interval

• Examples: Relative neutrophilia, Relative lymphopenia

• If one cell type's proportion increases → others must decrease (they are relative) → use absolute counts for accuracy

Step 4 — Calculate absolute counts (if not reported)

• Absolute Count = Relative Count × Total WBC Count

Step 5 — Examine each cell line for immature cells

Step 6 — Report morphologic abnormalities

• Cytoplasmic toxic granulation (seen in lead poisoning)

• Nuclear abnormalities (e.g., hypersegmentation)

• Some analyzers can't detect morphology → manual checking important

Terminology for WBC Changes:

Origin of "Left Shift"

• From Josef Arneth (1920s) — correlated neutrophil maturity with segment count

• Fewer segments = farther left on graph = left shift

• Called Arneth Count / Arneth-Schilling Count

• Abandoned ~1955 (when Arneth died), but term persisted

• Now means: ↑ immature neutrophilic cells = indicator of infection

RBC PARAMETERS

RBC-Related Parameters:

RBC Indices Formulas (Sir Liwana):

MCHC = HGB / HCT × 100
MCH = HGB / RBC × 10
MCV = HCT / RBC × 10

Steps in RBC Interpretation:

Step 1 — Examine HGB (or HCT) for anemia or polycythemia

• Use age-and-sex-specific reference intervals

• Rule of Three: HCT = HGB × 3 (for normochromic, normocytic RBCs)

  • If rule of three fails → possible analytical error (falsely ↑ or ↓ Hgb or Hct)

• HGB = more reliable indicator (direct O₂ carrying capacity)

• HCT = can be influenced by RBC volume (e.g., dehydration)

Step 2 — Evaluate MCV (average RBC volume)

Microcytic Anemia — Mnemonic: TAILS

• Thalassemia

• Anemia of Chronic Disease

• Iron Deficiency Anemia

• Lead Poisoning

• Sideroblastic Anemia

Macrocytic Anemia → usually Megaloblastic Anemia (Folate & Vit B12 deficiency)

• VitB12 deficiency cause: Diphyllobothrium latum (fish tapeworm)

• VitB12 malabsorption cause: Fasciolopsis buski

Normocytic → check reticulocyte count for underlying abnormality

Step 3 — Examine MCHC (hemoglobin filling of cells)

2 Situations of Elevated MCHC:

1. Spherocytes (>36 g/dL) — decreased surface area, no central pallor, Hgb more concentrated

2. Falsely elevated Hgb (~60 g/dL) — due to lipemia, icterus, or grossly elevated WBC → interferes with spectrophotometric Hgb measurement

• ↓ MCHC → Hypochromic

• ↑ MCHC → Spherocytosis (or analytical error)

• MCHC is best used as an internal quality control parameter

Step 4 — Evaluate RDW (anisocytosis indicator)

• From histogram of RBC volumes

• Reference interval: 11.5–14.5%

• Narrow histogram = uniform RBC size

• Wide histogram = variable RBC size (anisocytosis)

• Only increased RDW values have clinical significance

• Low RDW → relatively normal size distribution

• Reported as CV (coefficient of variation) or SD (standard deviation)

• MCV alone isn't enough; cells may have the same average but wide variation in size

Step 5 — Examine relative & absolute reticulocyte counts (if ordered)

• Assesses erythropoietic activity of bone marrow

• Anemia + ↓ reticulocyte count → poor BM response → investigate BM failure

• IRF (Immature Reticulocyte Fraction) = sensitive measure of erythropoietic activity

• Reticulocyte count is NOT a standard part of CBC (ordered separately, same analyzer)

Step 6 — Examine RBC morphology

• Abnormalities: volume, arrangement, color, shape, inclusions, immature RBCs

• If anemia is present → Wright-stained peripheral blood film MUST be reviewed

• Blood film = quality control (morphology seen on microscope must match analyzer results)

• Only record morphology when abnormalities are present

Step 7 — Examine RBC count and MCH

RBC count:

• NOT the primary parameter for judging anemia

• Some anemias have normal or elevated RBC count (e.g., thalassemia)

• Parameters for anemia = Hematocrit, Hemoglobin, Morphology (Rule of 3)

MCH:

• Less critical when other parameters are available

• Follows MCV: smaller cells = less Hgb; larger cells = more Hgb

• When MCH doesn't follow MCV → MCHC detects the discrepancy

PLATELET PARAMETERS

Platelet-Related Parameters:

Steps in Platelet Interpretation:

Step 1 — Examine platelet count

• ↑ = Thrombocytosis

• ↓ = Thrombocytopenia (may cause unexplained bruising/bleeding)

• Assess platelet count with WBC and HGB:

  • Pancytopenia → may suggest acute leukemia or aplastic anemia

  • Pancytosis → associated with polycythemia vera

Step 2 — Compare MPV with reference interval

• Reference: 6.9–10.2 fL

• ↑ MPV = ↑ platelet diameter on PBS

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Step 3 — Examine platelet morphology & arrangement

• Large platelets = 2× normal size

• Giant platelets = >2× normal size (or compare with RBC diameter)

• Granularity:

  • Reduced → hypogranular

  • Absent → agranular

• Variable, unclassifiable abnormalities → described as "bizarre" or dysplastic

• Platelet satellitosis (platelets attached to WBCs) → affects accurate platelet counts — must be noted

CORRECTED WBC (for NRBCs)

$\text{Corrected WBC}=\frac{\text{Observed WBC}\times100}{100+\text{\# NRBC}}$

Example:

• 12 NRBCs per 100 WBCs; Total WBC = 24 × 10⁹/L

• Corrected WBC = (24 × 100) / (100 + 12) = 21.4 × 10⁹/L

CASE STUDY ANSWER

Q: Normal MCV, normal MCHC, elevated RDW, peripheral smear shows size variation.

A: Anisocytosis — RBCs are normocytic and normochromic, but size variation (anisocytosis) is present

QUICK MEMORY AIDS

Good luck on your exam!