Complete blood count: what it includes and how to read each line

It is the test that almost always comes first in any check-up^[1] and, even so, the least understood. The complete blood count arrives with some twenty lines, each with its abbreviation, its number and its range, and one asterisk is enough for the rest of the day to go into searching what it means. This text walks through that blood test line by line: what each one measures, what the doctor looks at when it comes out low or high, which lines are read together and what it does not include even though many people think it does. In many countries the same test is called a hemogram; the paper is the same.

Before reading anything, what cannot wait: very low hemoglobin with shortness of breath or chest pain, very low platelets with bruises without a blow or bleeding that does not stop, or very high or very low leukocytes with fever. If that is you, do not wait for next month’s appointment: seek care today. Laboratories flag those results as “critical values” for exactly this reason.

What the blood count includes (and what it does not)

The complete blood count counts and describes the cells of the blood in three groups: the red series (red blood cells, which carry oxygen from the lungs to the rest of the body), the white series (white blood cells, which fight infections) and platelets (fragments that help the blood clot).^[1] It is a routine part of a check-up and also serves to follow infections, anemias, immune system problems and treatments that affect the blood.^[1]

Just as important is what it does not measure. The blood count does not include ferritin (iron stores), glucose, TSH (thyroid), cholesterol or creatinine. Those are separate tests, even if they are drawn in the same visit. If your question is glucose, kidneys or lipids, the answer is in the blood chemistry panel. And if the question is iron, the blood count only gives clues: hemoglobin and hematocrit are “neither sensitive nor specific” for iron, and the stores are measured with ferritin, which has to be ordered explicitly.^[9]

Red series: hemoglobin and what surrounds it

Hemoglobin and hematocrit

Hemoglobin (Hb) is the iron-rich protein that, inside red blood cells, carries oxygen.^[1] It is the line that defines anemia: when it drops below a certain threshold, which depends on sex, age, pregnancy and altitude, we speak of anemia.^[7] Hematocrit (Hct) is the percentage of the blood taken up by red cells.^[1] The two almost always rise and fall together, which is why they are read as a pair: low hemoglobin with low hematocrit tells the same story twice.

When they come out low, the doctor thinks first of anemia and its cause: blood loss, too little iron, lack of vitamins, an underlying disease.^[1] Tiredness, dizziness, cold hands and pallor are the usual way it shows up; in the most intense cases shortness of breath and chest pain appear.^[11] When they come out high, the most frequent cause is the simplest: dehydration, because with less water the blood is more concentrated.^[1] They also run higher, quite normally, in people who live at high altitude and in smokers.^[8]

Red cells (red blood cell count)

It is the number of red blood cells per unit of blood. It almost always moves with hemoglobin; its interest lies in the exceptions (many small, pale red cells, or few but large ones), and reading those takes the indices.

The indices: MCV, MCH, MCHC and RDW

The indices describe what the red cells are like, not how many there are. MCV (mean corpuscular volume) is the average size of each red cell; MCH (mean corpuscular hemoglobin), how much hemoglobin each one carries; MCHC, how concentrated that hemoglobin is inside the cell; and RDW (red cell distribution width), how similar the red cells are to each other in size.^[2]

Here is the reading most worth learning. A low MCV (small red cells) with low MCH points to iron deficiency or thalassemia; a high MCV (large red cells) points to vitamin B12 or folate deficiency, or to liver disease.^[2] RDW refines the clue: small red cells that are very unequal to each other (RDW above 14 %) is the typical pattern of iron deficiency; small but all alike looks more like thalassemia minor.^[8] On the high-MCV side, B12 deficiency is more common in older adults, in vegetarian or vegan diets and in people who take metformin or acid blockers for a long time, and it can cause tingling in the hands and feet even without anemia.^[10] None of the indices works alone; of RDW in particular, the US National Library of Medicine warns that it “is not enough for your provider to make a diagnosis”.^[2]

White series: leukocytes and the differential

Leukocytes or white blood cells are the defense cells. The blood count gives the total first and then the differential: how much belongs to each of the five types.^[1] A high total usually signals infection or a reaction to a medicine;^[1] inflammation, autoimmune diseases, allergy and, less often, blood diseases also raise it.^[3] A low total makes the doctor think of the bone marrow, an autoimmune disease, certain medicines (chemotherapy is the typical example, also some antibiotics) or infections such as HIV.^[3]

• Neutrophils: the main defense when bacteria, viruses or other germs get in. They are the ones that usually move the total in an ongoing infection.^[3]
• Lymphocytes: include B cells, which make antibodies against viruses, bacteria and toxins, and T cells.^[3]
• Monocytes: kill bacteria, viruses and other germs that can make you sick.^[3]
• Eosinophils: defend against parasites and infections, and take part in inflammation and allergy.^[3]
• Basophils: release enzymes during allergic reactions and asthma attacks.^[3]

The differential adds the nuance: a high total due to neutrophils is not the same as one due to eosinophils, and the doctor looks at which of the five lines moved the total. An abnormal result “doesn’t always mean you have a condition that needs medical treatment”; a recent injury, extreme stress, smoking and alcohol also move these numbers.^[3]

Platelets and MPV

Platelets are essential for clotting: they gather where there is a wound and, with blood proteins, form the clot that stops the bleeding.^[4] When they come out low (thrombocytopenia), you may bleed more than normal after a cut; behind it there may be viral infections, autoimmune diseases, vitamin B12 deficiency, bone marrow damage, cirrhosis or, in pregnancy, a mild form called gestational thrombocytopenia.^[4] When they come out high (thrombocytosis), the blood may clot more than it should; infections, inflammatory diseases, anemia and major blood loss cause it, among other things.^[4]

MPV (mean platelet volume) is the average size of the platelets. New ones are larger than old ones, so a high MPV suggests the marrow is making many new platelets, for example because the old ones are being destroyed quickly, and a low MPV, that it is making few.^[5] It is always read together with the count: “an MPV test alone can’t diagnose a health condition”.^[5]

Line by line: what it measures and what it is read with

Ranges change: sex, pregnancy, altitude and laboratory

The range printed next to your number is the laboratory’s, not yours. In the blood count this weighs more than in almost any other test, because the red series depends on sex, age, pregnancy and the altitude you live at.^[7] The clearest example is hemoglobin. The World Health Organization published in 2024 an updated guideline with the thresholds that define anemia,^[6] different for each group:^[7]

WHO works in grams per litre (120 g/L); laboratories in Mexico usually report in grams per decilitre (12.0 g/dL). It is the same number with the decimal point moved one place.

Altitude changes the reading. Higher up there is less oxygen and the body responds with more hemoglobin, so in a high city hemoglobin is, quite normally, higher than at sea level. The WHO guideline gives adjustments that are subtracted from the measured value before comparing it with the threshold: 0.8 g/dL between 1,000 and 1,499 metres, 1.1 g/dL between 1,500 and 1,999, 1.4 g/dL between 2,000 and 2,499 and 1.8 g/dL between 2,500 and 2,999.^[7] In practice, a hemoglobin of 12.5 g/dL in a woman living in Mexico City, above 2,000 metres, is equivalent to about 11.1 g/dL at sea level, and that is already anemia territory. Smoking also raises it, by about 0.3 g/dL,^[8] and the doctor takes that into account.

In pregnancy, blood volume and the number of red cells change,^[9] and the anemia threshold drops, especially in the second trimester.^[7] Platelets can also drop a little without meaning disease.^[4] And there are same-day factors that move any line: diet, physical activity, medicines, being on your period and having drunk little water.^[1] For all these reasons, the laboratory range is a starting point, not your personal norm.

A value slightly out of range: what usually happens next

An asterisk next to a line is not a diagnosis. The US guideline on iron deficiency says it plainly: a positive screening result is repeated to confirm it before acting.^[8] And “if any of your levels are abnormal, it doesn’t always mean you have a medical condition that needs treatment”.^[1] What follows, almost always, is one of these three things:

1. Repeat the blood count in a few weeks, under similar conditions (same time of day, well hydrated, not during an infection), to see whether the value holds.
2. Read it in context: the flagged line with the ones that go with it (hemoglobin with MCV and RDW, leukocytes with the differential, platelets with MPV) and with what was going on that day: fever, period, little water, a new medicine.
3. Add the missing test: ferritin if the clue is iron, vitamin B12 and folate if the MCV is high. That decision is your doctor’s.

The most useful comparison is not with the table but with yourself: a hemoglobin that has sat at the same point for years tells a different story from one that dropped a gram since last year. That is why it is worth keeping every blood count, with the date and what was happening that day, and bringing all of them to the appointment, not just the last one. If reading a report without panic is still hard, this guide comes first.

Warning signs: when you do not log, you consult

The blood count is, most of the time, a calm test. These combinations are the exception and call for medical care today, not next week:

• Very low hemoglobin together with shortness of breath, chest pain, palpitations or dizziness on standing up: these are the signs of intense anemia.^[11]
• Very low platelets with bruises that appear on their own, red pinpoint spots on the skin, nosebleeds or bleeding gums that do not stop, a period much heavier than usual, or blood in urine or stool.^[4]
• Very high or very low leukocytes together with fever, especially if you take medicines that lower your defenses or are on chemotherapy.^[3]
• Any line the laboratory has flagged as a critical value, or a result the laboratory itself called you about.