Understanding your complete blood count (CBC): what every number means

A complete blood count — or CBC — measures the cells in your blood across three main groups: red blood cells, white blood cells and platelets. It's one of the most routinely ordered tests in medicine because it gives a broad picture of your overall health and flags a wide range of conditions early.

Reading your own results can feel overwhelming, but the logic behind each value is straightforward once you know what you're looking at.

Red blood cell (RBC) values

Red blood cells carry oxygen from your lungs to every tissue in your body. Several related measurements appear on a CBC report:

• **RBC count** — the actual number of red cells per litre of blood. Low counts can indicate anaemia or bleeding; high counts may reflect dehydration or certain bone marrow disorders.
• **Haemoglobin (Hb or Hgb)** — the protein inside red cells that binds to oxygen. This is the most clinically important red cell value. Normal ranges differ by sex: roughly 130–175 g/L for men and 115–155 g/L for women.
• **Haematocrit (Hct)** — the percentage of your blood volume made up of red cells. Broadly tracks with haemoglobin.
• **MCV (mean corpuscular volume)** — the average size of your red cells. Small cells (low MCV) often indicate iron deficiency; large cells (high MCV) can suggest B12 or folate deficiency.
• **MCH and MCHC** — measures of haemoglobin content per cell, used mainly to classify different types of anaemia.

White blood cell (WBC) values

White blood cells are your immune system's army. A total WBC count tells you if that army is unusually large (often suggesting infection or inflammation) or unusually small (which can mean the immune system is suppressed). A WBC differential goes further, breaking the count down by cell type:

• **Neutrophils** — the first responders to bacterial infection. A high neutrophil count almost always means the body is fighting something.
• **Lymphocytes** — the specialists that handle viral infections and coordinate longer-term immunity. High counts can accompany viral illness; very low counts may indicate immune suppression.
• **Monocytes** — involved in clearing debris and chronic inflammation.
• **Eosinophils** — elevated in allergic reactions and parasitic infections.
• **Basophils** — least common; involved in allergic and inflammatory responses.

Platelet count and MPV

Platelets are tiny cell fragments that clump together to stop bleeding. A normal platelet count is roughly 150,000–400,000 per microlitre. Very low counts (thrombocytopenia) increase bleeding risk; very high counts can increase clot risk.

MPV (mean platelet volume) indicates the average size of platelets. Larger platelets tend to be more active. A high MPV combined with a low platelet count can suggest the bone marrow is working hard to compensate for platelet destruction.

What 'out of range' actually means

Reference ranges represent the middle 95% of a healthy population — which means 5% of perfectly healthy people will have at least one value outside the range on any given day. A single out-of-range result is not automatically a problem. Context matters enormously: your age, sex, altitude, recent illness, hydration level and dozens of other factors shift where your normal sits.

What's most useful is tracking the same values over time. A haemoglobin that has dropped steadily over four tests tells a different story from one that's always been at the low end of normal. Looms tracks your lab result trends automatically and flags values moving in the wrong direction, so patterns don't get lost between appointments.

"A single CBC tells me more about a patient in two minutes than a ten-minute conversation. The numbers are the story — you just need to know how to read them."

Dr. Lina Haddad, CMO, Looms

When to contact your doctor

• Any result flagged as 'critical' — laboratories add this label when a value is dangerous enough to require immediate clinical attention
• A result that is significantly outside the reference range, not just borderline
• A trend in the wrong direction across two or more consecutive tests
• Symptoms that match what the numbers might suggest — fatigue with a low haemoglobin, unusual bruising with a low platelet count
• Any result you simply don't understand after reading the report

The bottom line

A CBC gives you a window into your health that wasn't available to previous generations. Understanding the basics means you can participate in conversations about your own care rather than just receiving instructions. The more context you bring to those conversations, the better the decisions you and your doctor can make together.