Anaemia blood tests: a practical investigation guide

An “anaemia panel” is not one fixed package. Anaemia means that haemoglobin is below the appropriate limit for the person’s age, sex and physiological context, but that finding does not identify the cause. A useful investigation starts with the full blood count (FBC; also called a complete blood count or CBC), then adds targeted tests according to the red-cell pattern, symptoms, history and examination.

The sequence matters. Iron deficiency, vitamin B12 or folate deficiency, inflammation, kidney disease, blood loss, haemolysis, inherited conditions and bone-marrow disorders can overlap or look similar at first. A normal haemoglobin does not exclude depleted iron stores or vitamin B12 deficiency, and NICE specifically advises not ruling out B12 deficiency solely because anaemia or macrocytosis is absent.

Start with the full blood count

The FBC provides the first map, not the final diagnosis:

• Haemoglobin (Hb) reflects the blood’s oxygen-carrying protein and establishes whether the laboratory defines the result as anaemia.
• Haematocrit (Hct) is the proportion of blood occupied by red cells. Hydration can shift it, so it supports rather than replaces Hb.
• Red blood cell count (RBC) shows the number of circulating erythrocytes. A relatively preserved or high RBC count despite a low MCV can point away from straightforward iron deficiency and towards an inherited microcytosis such as thalassaemia trait.
• MCV describes average red-cell size; MCH describes the average amount of haemoglobin per cell.
• RDW describes variation in red-cell size. A rise can be an early clue or show a mixed population, but it is not specific enough to diagnose the cause.

Read the exact units and reference intervals printed by the laboratory. Hb and Hct can be affected by pregnancy, altitude, smoking and hydration, while transfusion can temporarily blur several red-cell indices. Our haemoglobin guide explains Hb thresholds and limitations in more detail.

How MCV, MCH and RDW shape the next step

The table is a pattern guide for discussion with a clinician, not a self-diagnosis tool.

MCV is an average: simultaneous iron deficiency and B12/folate deficiency can produce a deceptively normal MCV. MCH often falls with iron-restricted red-cell production. RDW may rise when newly produced cells differ in size from older cells, including after treatment, but no one index proves the diagnosis.

Ferritin and the inflammation caveat

Ferritin is the single most useful marker of stored iron. A low result strongly supports iron deficiency, which can exist before Hb falls. Normal Hb therefore cannot be used to declare iron stores normal.

Ferritin is also an acute-phase protein. Infection, inflammation, liver disease and other conditions can raise it, creating an apparently normal or high result despite iron-restricted production. BSG guidance recommends considering other iron tests, particularly transferrin saturation, when a false-normal ferritin is suspected. CRP or another inflammation assessment may provide context; the appropriate threshold depends on the clinical setting and laboratory.

Transferrin saturation, TIBC and serum iron

Serum iron changes with timing, food and illness, so it should not be interpreted alone. Transferrin carries iron; total iron-binding capacity (TIBC) estimates the blood’s potential iron-binding capacity. Transferrin saturation (TSAT) is calculated from serum iron and transferrin or TIBC.

In uncomplicated iron deficiency, ferritin and TSAT tend to be low while transferrin or TIBC tends to rise. With inflammation, serum iron and TSAT may be low but transferrin/TIBC can be low or normal and ferritin may be misleadingly preserved. These patterns help distinguish possibilities; they do not replace investigation of why iron is low.

Vitamin B12, folate, MMA and homocysteine

Macrocytosis can suggest B12 or folate deficiency, but neither a normal MCV nor a normal Hb rules out B12 deficiency. Neurological B12 deficiency can occur without anaemia. NICE NG239 uses total B12 or active B12 as the usual initial test in adults, with different approaches in pregnancy or suspected nitrous-oxide-related deficiency.

When a total or active B12 result is indeterminate and symptoms or signs persist, methylmalonic acid (MMA) may help clarify functional B12 deficiency. Kidney impairment can raise MMA, so context matters. Homocysteine can rise with B12 or folate deficiency but is less specific and is affected by other factors. Folate is usually measured alongside B12 when a megaloblastic pattern is possible. See the detailed vitamin B12 and folate guide.

Do not self-treat a possible B12 deficiency with folic acid alone: it may improve the blood count while neurological injury continues. Suspected B12 deficiency with neurological symptoms requires prompt clinical assessment; NICE advises that treatment should not be delayed while awaiting results in suspected megaloblastic anaemia with neurological symptoms.

What the reticulocyte count adds

Reticulocytes are young red cells recently released by the bone marrow. The result asks whether marrow production is responding appropriately:

• Low or inappropriately normal reticulocytes in anaemia suggest underproduction, as may occur with iron/B12/folate deficiency, kidney disease, inflammation or marrow disorders.
• Raised reticulocytes suggest the marrow is responding to red-cell loss or destruction, such as bleeding or haemolysis, although the response may take several days and can be limited when deficiencies coexist.

Clinicians may use a corrected reticulocyte count or reticulocyte production index because a raw percentage can look falsely reassuring when the total red-cell mass is low. A blood film, bilirubin, LDH and haptoglobin may be added if haemolysis is suspected.

Common combinations, not fixed diagnoses

Patterns can overlap. Treatment itself, recent bleeding, transfusion, pregnancy and mixed deficiencies can change the expected combination. The purpose of the panel is to narrow the differential diagnosis, not to generate an automatic label.

Why finding the cause matters

Replacing iron or vitamins without identifying the source can delay diagnosis. Iron deficiency may follow heavy menstrual bleeding, pregnancy, reduced intake, coeliac disease or other malabsorption, but it may also reflect gastrointestinal blood loss. BSG guidance emphasises appropriate investigation of confirmed iron-deficiency anaemia, particularly unexplained disease in men and postmenopausal women, because significant gastrointestinal pathology can present without specific symptoms.

Iron or B12 courses often run for months — tracking medication supply and refills helps avoid an unplanned gap in treatment before a follow-up test confirms the response.

The history should cover menstrual and other bleeding, black stools, bowel change, abdominal symptoms, diet, pregnancy, medicines, gastrointestinal surgery, alcohol, chronic inflammatory or kidney disease, family history and neurological symptoms. The clinician may add coeliac screening, kidney/liver/thyroid tests, a blood film, haemoglobin analysis or investigations for bleeding. Iron or B12 supplements should not be used as a substitute for that evaluation.

When to seek urgent help

Seek emergency help for chest pain, severe breathlessness at rest, fainting, confusion, rapidly worsening weakness, vomiting blood, heavy uncontrolled bleeding or black tarry stools with marked illness. These symptoms matter more than waiting for another routine panel.

Arrange prompt clinical assessment for new numbness or pins and needles, loss of balance, gait change, limb weakness, visual disturbance or cognitive change when B12 deficiency is possible. Also contact a clinician promptly about unexplained anaemia, persistent bleeding, unintentional weight loss, a new bowel change, pregnancy with an abnormal result, or a rapidly falling Hb. Do not wait for anaemia to appear before seeking help for neurological red flags.

Tracking the work-up with HealthLab

HealthLab can keep laboratory PDF reports together and plot Hb, MCV, ferritin, TSAT, B12, folate and reticulocytes over time. Recording illness, inflammation, treatment, bleeding, pregnancy, transfusion or a change of laboratory beside a result helps make the trend more useful at an appointment.

Keep units, laboratory intervals and test names visible when comparing results. HealthLab organises the history; it does not diagnose the cause of anaemia, assess neurological symptoms or replace urgent medical care.

Frequently asked questions

This material does not replace medical advice. Anaemia and nutrient deficiencies require clinical interpretation and investigation of the cause.