Hematology Testing: How It Works & What Clinical Labs Need to Run It

Hematology is the study of blood, specifically the cells that make it up, the proteins that circulate in it, and the bone marrow responsible for producing it. Hematology testing is the clinical application of that study.

This diagnostic field is one of the busiest, most information-dense corners of clinical medicine. A single tube of blood run through a hematology lab can tell whether someone is anemic, fighting an infection, responding to chemotherapy, or developing a bleeding disorder. 

Some of these tests are performed daily as part of a routine physical. Others are ordered only when a doctor is chasing down a symptom. Either way, the workflow behind them relies on a set of lab tests and equipment, which is what this guide covers.

Complete Blood Count (CBC)

A complete blood count measures red blood cells, white blood cells, and platelets. It also calculates related values, such as hemoglobin, hematocrit, and mean corpuscular volume. CBC is almost always the first test that tells clinicians where to look next and rarely delivers a final diagnosis.

White Blood Cell Differential

A total white blood cell count is useful because it signals the immune system is responding to a stressor, but it can easily hide a major underlying imbalance. To reveal those hidden details, a white blood cell differential breaks the total white blood cell count down into its five component cell types (neutrophils, lymphocytes, monocytes, eosinophils, and basophils), helping clinicians determine the exact cause of an illness and guide the appropriate treatment. 

If the results show a spike in the sample's monocyte count, the patient may be suffering from a chronic inflammation, whereas high levels of eosinophils or basophils usually indicate a severe, active allergic reaction. In addition, high neutrophils suggest a bacterial infection, and elevated lymphocytes point to a viral infection.

A peripheral blood smear is not a commonly ordered stand-alone diagnostic test. It serves as a confirmatory test, typically paired with a CBC to investigate anomalous cell distributions or flagging patterns. In this test, a lab analyst thinly spreads a single drop of blood onto a slide, stains it, and then reviews the cells under a microscope. They can also run the blood sample through an automated imaging system for digital analysis.

Erythrocyte Sedimentation Rate (ESR)

The ESR or sed rate measures how fast red blood cells reach or settle to the bottom of a vertical tube within an hour. RBCs settle slowly under normal conditions, but during systemic inflammatory states, they sink quickly.

Like the CBC, the ESR does not diagnose a condition on its own. Instead, it merely indicates that an inflammatory process might be occurring. It is most commonly used to monitor active autoimmune diseases or help diagnose certain inflammatory conditions.

Reticulocyte Count

A reticulocyte count measures how many reticulocytes (immature red blood cells) are present in the blood sample. It assesses the real-time activity of the bone marrow: Is it hyperactive, or is there bone marrow failure?

This test is necessary if the sample shows abnormal red cell values on a CBC. It is also a standard check after chemotherapy or a bone marrow transplant.