Understanding a Blood Test: Parameters and Reference Ranges

You come back from the laboratory, put the report on the kitchen table, and see rows of numbers where arrows pointing up or down sit next to half the values. The first thought is usually the same: something is wrong. Let's sort this out without panic: what exactly is measured in a complete blood count and where the numbers called normal come from.

What a complete blood count consists of

A complete blood count is not a single test but a group of measurements. The analyzer counts the number and size of different cells: red blood cells, which carry oxygen from the lungs to the tissues; white blood cells, which fight infections and other diseases; and platelets, which help blood clot. Hemoglobin is measured separately — an iron-containing protein that carries oxygen — along with hematocrit and mean corpuscular volume.

An automated hematology analyzer produces from 5 to 24 parameters in a single run. Besides the main numbers, the report may include a leukocyte differential — a count of each of the five types of white blood cells — and the ESR. Blood is drawn on an empty stomach, from a finger or a vein; repeat results should be compared using the same type of specimen.

Red blood cells, hemoglobin, and hematocrit: what they show

The red part of the report is responsible for oxygen. Red blood cells (RBC) are given in reference ranges as 4.3–5.5 ×10¹² cells/L, hemoglobin (HGB) as 120–140 g/L, and hematocrit (HCT) as 0.39–0.49. Hematocrit shows the proportion of blood volume taken up by cells: roughly 40–45%, with the rest being plasma.

Next to them are the red cell indices. MCV, the mean corpuscular volume, is 80–95 fL. MCH, how much hemoglobin falls on average to one cell, is 27–31 pg. MCHC, the hemoglobin concentration in red blood cells, is 320–360 g/L; this parameter is the most stable, so it is used to catch an analyzer or sample preparation error.

Deviations in the red line can be a sign of dehydration, anemia, heart disease, or iron deficiency. The same arrow can appear both with disease and under the influence of diet — it is up to the doctor to sort out the causes.

White blood cells and the leukocyte differential

White cells are the immune guard. The total white blood cell count (WBC) is normally given as 4–9 ×10⁹ cells/L. The differential lists five main types: lymphocytes — 25–40% and 1.2–3.0 ×10⁹/L in absolute terms, monocytes — 4–11% and 0.1–0.6 ×10⁹/L, granulocytes — 47–72% and 1.2–6.8 ×10⁹/L, plus eosinophils and basophils.

A low white blood cell count can occur with autoimmune disorders, bone marrow diseases, and oncological diseases. A high count can occur with infection or as a reaction to a medication. The number alone decides nothing: the doctor looks at which cells specifically changed and by how much.

Platelets and ESR

Platelets (PLT) are blood cells that stop bleeding by helping blood clot. The reference range is 150–400 ×10⁹ cells/L. Platelet indices may be listed alongside.

ESR — the erythrocyte sedimentation rate — is part of the complete blood count; the test allows the leukogram and ESR to be reviewed. It is not a diagnosis but another number on the report.

Reference ranges and why they differ

The numbers on your report are the reference intervals of the specific laboratory; ask your doctor which reference points to use when assessing the result. Comparing your result with norms from a random table is pointless: look at the intervals next to your numbers.

There are also non-laboratory reasons for discrepancies: diet, physical activity, medication use, menstruation, insufficient fluid intake. Results are usually ready within a few days, usually within a few weeks. If no one called you, you have not been forgotten: in some institutions, patients are contacted only when there is something to discuss.

When deviations require a doctor's consultation

A deviation from the norm is not a verdict and is not always a sign of a disease that requires treatment. The doctor compares the report with the history, complaints, and examination and, if necessary, orders other tests. A complete blood count is only one tool among many.

Bring a few things to the appointment:

• the report with the intervals of your specific laboratory;
• a note on whether blood was drawn from a finger or a vein;
• a list of medications you are taking;
• your complaints and when they appeared;
• previous results, if they have been preserved.

If you have questions about the results or do not understand them, ask the doctor to explain them.

The numbers on the report alone are not a reason to call an ambulance. The numbers 103 and 112 are needed when signs of a life-threatening condition appear — for example, a stroke or a heart attack.

This material is for reference: it helps you read the report but does not replace a doctor's consultation, and the final explanation of your numbers remains with the doctor.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer