Smear, Analyzer, And Result Correlation

How Automated Analyzers Count Cells

Modern hematology analyzers count and size cells by two main principles. Impedance (Coulter principle) sizes cells by the resistance change as each cell passes through an aperture - larger cells displace more electrolyte and produce a bigger pulse. Optical/flow cytometry (light scatter) characterizes cells by forward scatter (size) and side scatter (granularity/complexity), allowing a five-part differential.

The analyzer measures some values directly and calculates others:

Because Hct is calculated from MCV here, anything that distorts MCV (cold agglutinins, hyperglycemia) distorts the Hct as well. A built-in quality check the exam expects you to know is the rule of three: in a normal, normochromic, normocytic specimen, Hgb x 3 should approximately equal the Hct, and RBC x 3 should approximately equal the Hgb. If these relationships fail, suspect either true abnormal morphology or a specimen/instrument problem (lipemia, cold agglutinins, a clot, or hemolysis) before releasing results.

Hemoglobin is measured spectrophotometrically after conversion to a stable pigment, so anything that turbidly scatters light - lipemia, very high white counts, or icterus - can falsely raise the measured hemoglobin and therefore the calculated MCHC.

When To Review A Smear

Analyzers generate flags (suspect blasts, immature granulocytes, NRBCs, platelet clumps, abnormal scattergrams). Laboratory review criteria then determine when a manual smear and differential are required. Triggers commonly include:

• WBC, platelet, or Hgb above/below set limits, or any critical value.
• A delta check failure (a result that differs implausibly from the patient's prior result).
• Instrument flags for blasts, atypical lymphocytes, or NRBCs.
• First-time abnormal results on a new patient.

Nucleated red blood cells (NRBCs) falsely raise the automated WBC count; the WBC must be corrected: corrected WBC = (uncorrected WBC x 100) / (100 + NRBCs per 100 WBCs). Many modern analyzers detect and auto-correct for NRBCs, but the exam still tests the manual formula, so know it. A second mandatory practice is the well-made smear and the systematic review of the feather edge and the monolayer: the differential is counted in the monolayer where cells barely touch, while platelet clumps, fibrin strands, and large abnormal cells migrate to the feather edge.

A properly made wedge smear has a smooth bullet shape with a feathered tail and no ridges or holes; too large a drop or too steep an angle makes the smear too thick to evaluate.

Spurious Results And Their Causes

The exam loves artifact recognition. Memorize these:

An MCHC above ~36-37 g/dL is physiologically improbable except in spherocytosis; a high MCHC otherwise should trigger a search for cold agglutinins, lipemia, or a clot. The MCHC is therefore one of the best internal flags on the CBC - the exam loves to give an MCHC of 39 g/dL and ask for the most likely cause.

The reflexes to know cold: red cell agglutination on the smear plus high MCV/MCHC means cold agglutinins (warm the specimen to 37 C and rerun); a milky plasma layer with falsely high Hgb and MCHC means lipemia (use a plasma blank or saline replacement); and a specimen with falsely low RBC, platelets, and Hgb may simply be clotted (inspect for fibrin and reject). For a high white count interfering with the hemoglobin reading, a centrifuged plasma blank corrects the turbidity.

Delta Checks And Critical Values

A delta check compares the current result with the same patient's previous result; an implausible swing (for example a hemoglobin that jumps from 14 to 8 g/dL overnight without a clinical reason) flags possible specimen mislabeling, wrong-patient draw, or instrument error and must be resolved before reporting. Critical (panic) values - such as a very low platelet count, a very low or very high hemoglobin, or a markedly prolonged PT/INR - require immediate verification and direct notification of the care provider with read-back documentation.

These quality steps are not bureaucratic filler; on the exam, the correct "what do you do next" answer for an extreme or discrepant value is almost always to verify the specimen and result and follow the notification protocol, not to release it blindly.

Result Correlation Discipline

Before releasing results, correlate three things: the CBC numbers, the smear morphology, and the clinical/prior data. If the analyzer reports MCV 78 fL but the smear shows obvious macrocytes, suspect an artifact or instrument issue. If platelets read 30 x10^9/L but clumps are visible, the count is spurious. This "does the picture match the numbers" check is the core competency tested in this lane.

Worked Example

An automated count reports platelets of 28 x10^9/L on an EDTA specimen, but the patient is asymptomatic and the smear shows large platelet clumps at the feather edge. The clumping indicates EDTA-induced pseudothrombocytopenia; the correct action is to recollect in sodium citrate and recount, not to report a critical low.

Common Traps

• Releasing an analyzer platelet count without checking the smear for clumps.
• Forgetting to correct the WBC for NRBCs.
• Trusting a high MCHC instead of suspecting cold agglutinins, lipemia, or a clotted sample.