Diagnosis and investigation of iron deficiency anaemia

Iron deficiency anaemia is diagnosed by a full blood examination and iron studies, particularly serum ferritin levels. Serum iron levels should not be used to diagnose iron deficiency.

Tests for the diagnosis of IDA

 

Full blood examination

Haemoglobin (Hb) level is used to determine the presence and severity of anaemia.

Anaemia is defined as a haemoglobin (Hb) concentration below the reference range for the laboratory performing the test. Alternatively, published cut-off levels may be used to define anaemia. The World Health Organization (WHO) defines anaemia as a Hb level below 130 g/L in men, 120 g/L in non-pregnant women and 110 g/L in pregnant women and preschool children.

A constellation of the following findings on full blood count is highly suggestive of iron deficiency anaemia, but MCV and MCH can be normal in early stages of iron deficiency:

  • anaemia
  • microcytosis (low MCV)
  • hypochromia (low MCH)

Iron deficiency can occur without anaemia, and ferritin is required to confirm diagnosis.

 

Serum ferritin

Ferritin is the diagnostic test of choice. Ferritin is also an acute phase protein and is elevated in inflammation, infection, liver disease and malignancy. This can result in misleading elevated ferritin levels in iron deficient patients with coexisting systemic illness. Markers such as C-reactive protein (CRP) may help identify coexisting inflammation.

Iron deficiency is diagnosed when ferritin is <30μg/L in adults and < 20μg/L in children.

When ferritin is not elevated as an acute phase reactant, a value >30μg/L is normal. Ferritin can be as high as 100μg/L in adults and 150μg/L in children with iron deficiency and inflammation.  

Functional iron deficiency, also known as anaemia of chronic disease, occurs when iron can’t be mobilised for erythropoiesis despite adequate stores. It’s most commonly seen in patients with chronic kidney disease, chronic heart failure or inflammatory conditions such as rheumatoid arthritis. Ferritin can be up to 100μg/L.  

 

Transferrin and transferrin saturation

Elevated transferrin and low transferrin saturation are suggestive of iron deficiency even in the presence of a normal or elevated ferritin.

A transferrin saturation of < 20% is useful to define low iron availability in both absolute and functional iron deficiency.

Serum iron is often reported but only reflects recent intake and is not useful in the diagnosis of iron deficiency.

 

Additional tests

Additional tests, such as soluble transferrin receptor and bone marrow biopsy, may be considered when the clinical features and haematology profile are suggestive of iron deficiency, but ferritin is normal. Consider consulting with a Pathologist or Haematologist before ordering these additional tests.

 

Interpretation of laboratory results

The following table provides guidance for interpretation of results of the additional tests that can be used to assess iron status.

  Iron deficiency without anaemia Iron deficiency anaemia (IDA) Functional iron deficiency IDA + functional iron deficiency Thalassaemia minor
Haemoglobin N ↓ (or N)
MCV N or ↓ N (or mildly ↓)
Serum ferritin N or ↑ ↓ or N N or ↑
Transferrin N or ↑ ↓ or N N or ↑ N
Transferrin saturation ↓ or N ↓ or N N or ↑

N = normal 

↓ = decreased; 

↑ = increased 

* may also be referred to as functional iron deficiency, i.e. when iron cannot be mobilised for erythropoiesis despite adequate stores.

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