4.4 Hematology: CBC, Hematocrit, ESR & Coagulation Testing

The Complete Blood Count

The complete blood count (CBC) is the most frequently ordered laboratory test in ambulatory practice. It is collected in a lavender-top tube containing EDTA (ethylenediaminetetraacetic acid), a chelating anticoagulant that binds calcium and preserves cell morphology. The tube is inverted 8 to 10 times immediately after collection; underfilling it over-concentrates the EDTA and shrinks red cells, producing a falsely low hematocrit.

Reference ranges vary between laboratories and by instrument, so the range printed on the report always governs. Values are also age-dependent: newborns run much higher hemoglobin and hematocrit than adults, and children run higher lymphocyte proportions.

The Rule of Three

In a normal, normocytic specimen, hemoglobin multiplied by three approximates the hematocrit. A hemoglobin of 14 g/dL should pair with a hematocrit near 42 percent. When the rule of three fails badly, suspect a pre-analytical problem first — a clotted or hemolyzed specimen, a short draw in an EDTA tube, or a sample drawn above an intravenous line — rather than assuming pathology. This is the single most useful bedside quality check a medical assistant can apply to a hematology result.

Microhematocrit

The Clinical Laboratory Improvement Amendments (CLIA)-waived microhematocrit is performed by filling two capillary tubes about three-quarters full, sealing one end with clay, and centrifuging at high speed for the interval the manufacturer specifies (commonly 3 to 5 minutes). The tubes are placed with sealed ends against the gasket and balanced opposite each other. After spinning, the buffy coat — the thin grayish-white layer of leukocytes and platelets between packed red cells and plasma — is excluded from the reading. Duplicate results should agree within about 2 percent.

The Differential and the Shift to the Left

The differential reports each leukocyte type as a percentage of the total white count. A useful memory device for descending normal abundance is "Never Let Monkeys Eat Bananas" — Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils.

A shift to the left is an increased proportion of immature neutrophils, called bands, released prematurely from the marrow. It signals an acute bacterial process and appears on the exam paired with an elevated total WBC and fever. The classic contrast is viral illness, which more often shows a normal or low total WBC with relative lymphocytosis.

Erythrocyte Sedimentation Rate

The erythrocyte sedimentation rate (ESR) measures how far red cells fall through plasma in one hour, reported in millimeters per hour. The Westergren method is the reference technique. Typical adult values are approximately 0 to 15 mm/hr for men and 0 to 20 mm/hr for women, rising with age.

ESR is a nonspecific marker of inflammation. It rises in infection, autoimmune disease, malignancy, and pregnancy, and it is used clinically to monitor known inflammatory conditions such as temporal arteritis and polymyalgia rheumatica rather than to diagnose. Procedurally, the tube must be filled exactly to the mark, held perfectly vertical in a draft-free rack away from vibration, and read at exactly 60 minutes — a tilted tube falsely elevates the result.

Coagulation Testing and INR

Prothrombin time (PT) evaluates the extrinsic and common coagulation pathways and is standardized across laboratories as the International Normalized Ratio (INR). It is the monitoring test for warfarin. Activated partial thromboplastin time (aPTT) evaluates the intrinsic pathway and monitors unfractionated heparin.

Coagulation specimens are drawn into a light-blue-top tube containing 3.2 percent sodium citrate, which binds calcium reversibly. The blood-to-anticoagulant ratio is fixed at 9:1, so the tube must be filled completely to the stated volume. A short draw over-dilutes the specimen with citrate and falsely prolongs the PT/INR — a result that could prompt an unnecessary warfarin dose reduction. Invert the tube 3 to 4 times, and when a light-blue tube is the first tube drawn with a winged (butterfly) set, draw a discard tube first to clear the air in the tubing so the fill volume is correct.

A typical therapeutic INR for atrial fibrillation or venous thromboembolism is 2.0 to 3.0, with a higher range for some mechanical heart valves. Any INR outside the provider's ordered range — and any critically elevated value — is routed to the provider the day it is resulted, not held for the next visit.

C-Reactive Protein

C-reactive protein (CRP) is the other acute-phase marker the content outline names alongside the ESR, and the two are frequently ordered together and confused.

CRP is synthesized by the liver in response to inflammatory cytokines. Its value on the exam is its kinetics: it rises within 4 to 6 hours of the inflammatory stimulus, peaks at roughly 48 hours, and falls quickly once the process resolves. The ESR, by contrast, lags by days at both ends because it depends on the slower turnover of plasma fibrinogen and globulins. That difference is why CRP is preferred for tracking a rapidly changing process such as post-operative infection, while ESR remains useful for monitoring chronic conditions like temporal arteritis.

A standard CRP below approximately 10 mg/L is generally considered normal, with markedly elevated values suggesting bacterial infection or significant inflammation. The high-sensitivity CRP (hs-CRP) assay is a different application of the same analyte used for cardiovascular risk stratification, where under 1 mg/L is low risk, 1 to 3 mg/L is average, and above 3 mg/L is high risk. The two are not interchangeable — ordering a standard CRP when hs-CRP was intended produces a result that cannot answer the cardiovascular question.

Like the ESR, CRP is nonspecific: it tells the provider that inflammation is present and how it is trending, not what is causing it. Report the number and let the provider interpret it.