Differentials, Blood Film Morphology, Indices, Hemoglobin Methods, and Hematocrit

A number that survives the analyzer still needs a slide. The blood film corroborates or challenges the instrument: it sorts flagged counts into real populations, exposes counting interference, and supplies the morphology that diagnosis still rests on. This page works the film systematically, then follows the CBC from differential through indices to the two primary measurements, hemoglobin and hematocrit, that anchor the whole panel.

Systematic microscopic film examination

Instrument flags, internal inconsistencies, and the clinical question together decide whether a film is reviewed. Typical triggers include suspected platelet clumping, nucleated red cells, variant or blast cells, a failed rule-of-three check, a count at odds with its film estimate, and specific questions such as suspected leukemia, unexplained anemia, or active infection. Before the slide touches the stage, Köhler illumination is set: light source centered, condenser raised and matched to the objective, iris opened for comfortable brightness.

The macroscopic glance comes first, and it warns.

Low power (10× objective, 100× total). Survey quality and distribution. White cells piled more than fourfold denser at the edges than in the monolayer, the snowplow effect, mean the film must be remade, and visible fibrin strands reject the specimen for recollection. Rouleaux, agglutination, and large abnormal cells are conspicuous here, and the usable counting area is located: the zone where red cells sit singly with minimal overlap.

Intermediate power (40× high-dry or 50× oil). Count white cells in 10 consecutive fields in the usable area, average them, and multiply by the laboratory’s validated factor to estimate the count.

Worked example. A smear averages 7 white cells per 40× field and the laboratory’s validated factor is 2,000: the estimate is 7 × 2,000 = 14,000/µL (14 × 10⁹/L).

Every microscope’s magnification-to-field-diameter relationship differs. The laboratory validates its own multiplication factor against roughly 30 in-control specimens and adopts the factor from its own measurements rather than borrowing a generic factor.¹

High power (100× oil, 1,000× total). The 100-cell differential is performed along a serpentine battlement track, several fields up, across, several fields down, across again. The track distributes the count evenly and specifically sweeps the side margins, where monocytes, reactive lymphocytes, and immature cells concentrate. Count 200 cells when the white count exceeds 40 × 10⁹/L, and 300 to 400 at 100 × 10⁹/L or higher, improving statistical precision at high totals.² The optimal counting zone lies between the thick heel and the feather edge, where erythrocytes sit singly with preserved central pallor, about 200 to 250 red cells per oil-immersion field at a normal count. A film that focuses sharply at 10× and 40× but not at 100× is almost certainly upside down, because oil-immersion optics cannot focus through glass; remove the oil and flip the slide.

Platelet estimate. In the same 100× fields, count platelets across 10 consecutive fields in the optimal zone and average, then multiply by 20,000.

Worked example. An average of 14 platelets per oil-immersion field gives 14 × 20,000 = 280,000/µL (280 × 10⁹/L).

In marked anemia or erythrocytosis the platelet-to-red-cell ratio no longer tracks that factor, so the estimate anchors to the red-cell count directly:

Estimated platelets = (average platelets/field × RBC count) ÷ 200, counting about 200 red cells per field in the optimal zone

Worked example. In an anemic specimen averaging 8 platelets per field with a red-cell count of 2.10 × 10¹²/L: (8 × 2.10 × 10¹²) ÷ 200 = 84 × 10⁹/L.

Either film estimate cross-checks the instrument count and exposes platelet clumping or count interference. Morphology comments follow the laboratory’s validated grading policy; Auer rods, when seen, are reported as present.

Red-cell morphology on the stained film

Red-cell evaluation addresses size, hemoglobinization, shape, inclusions, immaturity, and arrangement, and the microscopic impression must corroborate the analyzer indices. A mismatch demands investigation before either is reported.

Size and chromia. Uniform microcytosis or macrocytosis mirrors a decreased or increased MCV, and cell-to-cell size variation, anisocytosis, is the microscopic counterpart of an elevated RDW. Normal cells show central pallor occupying about one third of the cell diameter. Hypochromic cells show enlarged pallor from restricted hemoglobin loading in iron deficiency and thalassemia. A spherocyte that appears “hyperchromic” simply lacks central pallor: its volume fell while its hemoglobin content stayed the same, modestly raising its MCHC.

Polychromatophilic cells, larger and slate-blue-gray, are young reticulocytes and signal marrow response; their impression on the film alone is insufficient, so quantify with the reticulocyte count and its production index. Nucleated red cells escaping to blood in severe anemia signal marrow stress or infiltration; when the analyzer does not enumerate and subtract them automatically, apply the corrected-white-cell formula from the cell-count topic.

White-cell parameters

The white-cell panel is read in order: total count, relative differential, absolute differential, morphology.

1. Confirm the count against the histogram and scatterplot. Nucleated red-cell handling is platform specific.
2. Assess the total: leukocytosis or leukopenia, then identify which lineage drove the change.
3. Screen the relative differential against reference terminology.

4. Calculate absolute counts as percentage × total white count, because percentages are mechanically interdependent: a relative change in one line can be entirely an artifact of a real change in another.

Worked example. The white count is 18.0 × 10⁹/L with 76% neutrophils, 14% lymphocytes, 7% monocytes, 2% eosinophils, and 1% basophils. Absolute neutrophils = 0.76 × 18.0 = 13.7 × 10⁹/L, clearly elevated. Absolute lymphocytes = 0.14 × 18.0 = 2.5 × 10⁹/L, within the stated absolute interval despite the low percentage. The absolute counts show that the lymphocyte number is normal and that neutrophilia is real. The absolute neutrophil count specifically grades neutropenia risk in chemotherapy and infection contexts.

5. Note immature cells by name in any line, prolymphocyte, lymphoblast, promonocyte, monoblast; immature eosinophils and basophils are reported simply as immature.
6. Report morphology, toxic granulation, Döhle bodies, vacuolization, hypersegmentation, as a separate section from the count.

The current CLSI reference differential keeps segmented neutrophils and band forms as separate categories while recognizing that some instruments combine them. Whether a routine report displays bands separately or inside the neutrophil count follows the laboratory’s validated method and reporting policy.²

Red-cell parameters and indices

The red-cell panel is the red-cell count, hemoglobin, hematocrit, MCV, MCH, MCHC, RDW, and morphology, and it is read in a deliberate order.

1. Hemoglobin first, because it measures oxygen-carrying capacity directly and hematocrit depends on cell size. The rule of three provides the internal check: for normocytic, normochromic blood, 3 × hemoglobin (g/dL) approximates the hematocrit (%) within about 3.

   Hematocrit (%) is approximately (hemoglobin × 3) ± 3

   Worked example. Hemoglobin 12.0 g/dL centers the expected hematocrit at 36%, roughly 33 to 39%. A reported hematocrit of 35% fits; 44% does not, and prompts review for legitimate index effects or a spurious measurement.

2. MCV against the film and the classical classification.

   MCV	Term
   Below 80 fL	Microcytic
   80–100 fL	Normocytic
   Above 100 fL	Macrocytic

3. MCHC for chromia. Normal values run 32 to 36 g/dL. Below 32 g/dL is hypochromic. Mild elevation, 36 to 38 g/dL, accompanies spherocytosis, while a dramatic elevation, occasionally approaching 60 g/dL, is spurious, produced by lipemia, icterus, or marked leukocytosis interfering with the spectrophotometric hemoglobin measurement. The MCHC doubles as a built-in analytic quality-control flag.

4. RDW, the red-cell histogram coefficient of variation (typical reference interval about 11.5 to 14.5%), quantifies anisocytosis; only elevation is meaningful. MCV and RDW read together, a normal MCV with an elevated RDW suggesting a mixed or early population, carry more information than either alone.

5. Morphology corroborates the indices. Entirely normal film morphology takes no free-text comment; any comment corresponds to an observed abnormality.

6. Red-cell count and MCH last. The red-cell count alone never establishes anemia severity, because thalassemia classically keeps the count normal to high while hemoglobin falls, a diagnostically useful discordance. MCH tracks MCV, since smaller cells hold less hemoglobin, and when it does not, the MCHC exposes the mismatch.

Index formulas, each with its own example values:

MCV (fL) = (HCT % ÷ RBC ×10¹²/L) × 10 Worked example. Hematocrit 36%, red cells 4.20 × 10¹²/L: MCV = (36 ÷ 4.20) × 10 = 85.7 fL, normocytic.

MCH (pg) = (HGB g/dL × 10) ÷ RBC ×10¹²/L Worked example. Hemoglobin 11.2 g/dL, red cells 4.20 × 10¹²/L: MCH = (11.2 × 10) ÷ 4.20 = 26.7 pg (adult interval about 26 to 32 pg; the MCH is never used alone to classify anemia).

MCHC (g/dL) = (HGB g/dL × 100) ÷ HCT % Worked example. Hemoglobin 11.2 g/dL, hematocrit 36%: MCHC = (11.2 × 100) ÷ 36 = 31.1 g/dL, just below the 32 g/dL limit, mildly hypochromic.

Integrated example. A patient has hemoglobin 9.8 g/dL, hematocrit 31%, red cells 3.50 × 10¹²/L, MCV 88.6 fL, MCH 28.0 pg, MCHC 31.6 g/dL, and RDW-CV 16.1%. The rule of three centers the expected hematocrit at 29.4%, compatible with the reported 31%. The MCV is normocytic, the MCHC is borderline low, and the RDW is increased: the pattern points toward an early or mild iron-restricted process, before a clear microcytic or macrocytic classification emerges.

Quantitative hemoglobin: the cyanmethemoglobin method

CLSI H15-A3, though archived from active revision, remains a technically valid reference procedure for hemoglobin concentration by the cyanmethemoglobin (hemiglobincyanide) method.⁴ Blood is diluted into alkaline Drabkin reagent, potassium ferricyanide, potassium cyanide, sodium bicarbonate, and a surfactant. Ferricyanide oxidizes hemoglobin iron from ferrous to ferric, converting every circulating hemoglobin species to methemoglobin; cyanide then binds the methemoglobin to stable cyanmethemoglobin, whose absorbance at 540 nm is directly proportional to hemoglobin concentration. Sulfhemoglobin alone resists conversion and goes unmeasured; clinically significant sulfhemoglobinemia is rare.

Procedure in brief. Build a standard curve from a certified cyanmethemoglobin standard, rebuilt with each new reagent lot or after any spectrophotometer change; dilute patient or control blood 1:251 into reagent; allow 10 minutes for complete conversion; read at 540 nm against a reagent blank and convert to concentration from the curve.

Sources of error. The reagent is light-sensitive, so it is stored in the dark or in an amber container. Turbidity from marked leukocytosis (above 20 × 10⁹/L), thrombocytosis (above 700 × 10⁹/L), or lipemia falsely elevates the reading; centrifuge and read the supernatant, or prepare a patient plasma blank. Cells containing Hb S or Hb C resist lysis and add turbidity; dilute 1:2 further with water and double the result. Paraproteins from myeloma or Waldenström macroglobulinemia may precipitate in the reagent; added potassium carbonate corrects this, or a modified low-precipitation reagent is used. The reagent contains cyanide and is never discarded down a drain; it requires licensed hazardous-waste disposal.

Handheld point-of-care devices avoid the cyanide hazard by forming azidemethemoglobin and reading it at two wavelengths to cancel turbidity, and some laboratory analyzers lyse with sodium lauryl sulfate to form SLS-methemoglobin, a nontoxic alternative chemistry.

Hemoglobin variant testing

Quantitative hemoglobin measurement answers how much; variant testing answers which hemoglobins. Hb S solubility testing is a screen for detectable Hb S, and positive results require confirmatory fractionation; evaluation of other hemoglobinopathies and thalassemias begins with fractionation and, when needed, molecular testing.

Hemoglobin solubility (sickle) screen. Whole blood is lysed with saponin in a buffered solution containing sodium dithionite, a reducing agent that drives hemoglobin iron into the deoxygenated, oxygen-binding-incompetent state. Deoxygenated Hb S polymerizes and turns the solution turbid; lines behind the tube become invisible, while solutions of non-sickling hemoglobins stay clear. The screen cannot separate Hb S trait from Hb S disease, and false negatives occur in severe anemia, too little hemoglobin to produce visible turbidity, and in very young infants, too little Hb S relative to the large Hb F fraction.

Confirmatory and quantitative methods compared:

A negative solubility screen with normal fractionation excludes detectable Hb S only when the patient’s age, hemoglobin concentration, transfusion history, expected Hb S fraction, and assay sensitivity make the tests reliable; infants, severe anemia, low Hb S fractions, and recent transfusion demand definitive fractionation and, when needed, molecular testing. A positive solubility test always proceeds to electrophoresis or HPLC to separate sickle cell trait (Hb AS) from sickle cell disease (Hb SS or a compound heterozygote such as Hb SC or Hb S-β-thalassemia).

β-thalassemia minor and iron deficiency anemia share a microcytic, hypochromic pattern and require separation:

Iron deficiency independently lowers hemoglobin A2, so iron stores are corrected before Hb A2 is used to evaluate for β-thalassemia trait. No single index separates the two conditions alone; the complete count, film, and iron studies are read together.

Microhematocrit

The hematocrit is the fraction of whole blood occupied by packed erythrocytes, the packed cell volume, reported as a percentage or a fraction, 36% or 0.36 L/L.

Procedure. Fill a plain capillary tube (anticoagulated venous blood) or a heparinized tube (skin puncture) about three quarters full, seal the dry end with clay in a plug of at least 4 mm, and centrifuge at 10,000 to 15,000 × g for the laboratory’s established maximum packing time. The laboratory sets that time by spinning duplicate specimens, one with a hematocrit at or above 50%, in 30-second increments from 2 minutes until two consecutive readings agree, and then adopts that time. Read the packed red-cell column on a microhematocrit reader, excluding the buffy coat; duplicates agree within 1% (0.01 L/L).⁵

Sources of error. A leaking seal falsely lowers the result, because erythrocytes are lost preferentially from the tube bottom. Excess anticoagulant from a short-drawn tube shrinks cells and lowers the reading. Under-centrifugation, a delayed reading, or inclusion of the buffy coat falsely raises it. After acute hemorrhage the hematocrit may initially hold and then fall as plasma volume is restored, and dehydration raises it through hemoconcentration. Sickle cell anemia, macrocytic and hypochromic anemias, spherocytosis, and thalassemia trap plasma within the packed column, so the spun hematocrit reads 1 to 3% (0.01 to 0.03 L/L) higher than a calculated or optically measured automated value, which experiences no trapped plasma.¹

References

1. Keohane EM, Preston MM, Mirza KM, Walenga JM, eds. Rodak's Hematology: Clinical Principles and Applications. 7th ed. Elsevier; 2025. Accessed August 31, 2026.
2. Clinical and Laboratory Standards Institute. Reference Leukocyte Differential Count (Proportional) and Evaluation of Instrumental Methods. CLSI document H20-Ed3. Wayne, PA: CLSI; 2026.
3. Zini G, d'Onofrio G, Briggs C, et al. ICSH recommendations for identification, diagnostic value, and quantitation of schistocytes. Int J Lab Hematol. 2012;34(2):107-116. doi:10.1111/j.1751-553X.2011.01380.x.
4. Clinical and Laboratory Standards Institute. Reference and Selected Procedures for the Quantitative Determination of Hemoglobin in Blood; Approved Standard. CLSI document H15-A3. Wayne, PA: CLSI; 2000.
5. Clinical and Laboratory Standards Institute. Procedure for Determining Packed Cell Volume by the Microhematocrit Method; Approved Standard. CLSI document H07-A3. Wayne, PA: CLSI; 2000.