Hematology

Why it matters

The complete blood count (CBC) is one of the most ordered tests in medicine. Hematology analysers count and size hundreds of thousands of cells in under a minute, but they cannot see everything. A large part of the hematology MLS's skill is knowing when an analyser's numbers need a blood smear review under the microscope, and what the smear shows.

Scope. The numbers in this lesson are illustrative teaching values, not reference ranges to apply to anyone. Every laboratory establishes and validates its own reference intervals for its methods and population. Interpreting results for a patient is the care team's job; nothing here is medical advice.

What the analyser measures

Analysers use electrical impedance (cells passing through an aperture change its resistance: the pulse size gives cell volume) and flow cytometry with light scatter and fluorescence to separate white-cell types. From these they report:

The differential splits white cells into neutrophils, lymphocytes, monocytes, eosinophils and basophils, as percentages and absolute counts. (All values above are illustrative only.)

The red-cell indices

Two indices are calculated from the measured values, and you can check them by hand:

• MCV (fL) = Hct (%) x 10 / RBC (x10^12/L)
• MCHC (g/dL) = Hgb (g/dL) x 100 / Hct (%)

A quick plausibility check, the rule of three, says that for normal-sized cells, Hgb x 3 is roughly the Hct. When it fails badly, think of interference: a cold agglutinin clumps red cells and gives a falsely low RBC and a falsely high MCV and MCHC; lipemia falsely raises hemoglobin.

Worked example: reading a CBC

A fictional result: Hgb 9.0 g/dL, Hct 29 %, RBC 4.6 x10^12/L, RDW 17 %, PLT 410.

• MCV = 29 x 10 / 4.6 = 63 fL: small cells (microcytic).
• MCHC = 9.0 x 100 / 29 = 31 g/dL: slightly low (hypochromic).
• RDW raised: the cells vary in size.

Small, pale, varied cells fit the classic laboratory picture of iron deficiency, while thalassemia trait often shows small cells with a normal RDW and a relatively high RBC count. Telling them apart needs further tests (iron studies, hemoglobin electrophoresis) ordered by the care team. The lab's job is an accurate count, correct flags and a good smear review.

When to look at a smear

Labs set smear review criteria: for example a first-time abnormal result, a very high or low count, or an analyser flag such as "blasts?", "atypical lymphs?", "platelet clumps" or "RBC fragments." On a stained peripheral smear, the MLS looks for:

• Platelet clumps: EDTA can make platelets clump in some people, giving a falsely low count (pseudothrombocytopenia). A redraw in a citrate tube often resolves it.
• Schistocytes: fragmented red cells, which can signal red cells being sheared in small vessels.
• Spherocytes, target cells, sickle cells: shape changes linked to specific conditions.
• Hypersegmented neutrophils (six or more lobes), often seen with B12 or folate deficiency and large cells.
• Blasts: immature white cells that must be reported and referred for pathologist review.

Any suspicion of blasts or a new serious finding follows the lab's escalation path; it is never left in a comment field alone.

Common mistakes

• Reporting a low platelet count without checking for clumps.
• Ignoring a failed rule of three.
• Running a CBC on a clotted tube.
• Reading a smear in the thick part of the slide, where cells overlap; the monolayer near the feathered edge is where morphology is reliable.

Quick recap

• Analysers use impedance and flow cytometry; flags trigger smear review.
• MCV = Hct x 10 / RBC; MCHC = Hgb x 100 / Hct; rule of three: Hgb x 3 is about Hct.
• MCV sorts anemias into microcytic, normocytic and macrocytic patterns for the care team.
• Smears catch platelet clumps, fragments, shape changes and blasts.

Try it

Open the Do tab for the quiz: calculate indices and decide when a smear is needed.