Immune thrombocytopenia: Why is diagnosis sometimes difficult?

What are the first signs that lead to a diagnosis?

ITP is sometimes discovered by chance, during a blood test carried out for a completely unrelated reason, in someone who has no visible bleeding. Most often, however, clinical signs prompt someone to seek medical advice and have a blood count: bruises that appear without a knock or seem disproportionate, purpura in the form of small red or purple spots on the skin, bleeding from the gums or nose, or, in women, unusually heavy periods.

Severe bleeding, particularly gastrointestinal or cerebral bleeding, remains rare and generally only occurs when the platelet count is very low, usually below 10,000/mm³.

The platelet threshold that triggers investigations

A diagnosis of ITP is considered when there is isolated thrombocytopenia, meaning a platelet count below 100 G/L, confirmed by a second blood sample to rule out false thrombocytopenia caused by a simple laboratory artifact.

The clinical examination should otherwise be normal, apart from any signs of bleeding. The absence of swollen lymph nodes or an enlarged spleen points towards so-called primary ITP, as opposed to forms secondary to another condition.

Why ITP is a diagnosis of exclusion

This is where the main diagnostic difficulty lies: there is no biological marker that can directly confirm ITP. The diagnosis is only made after other causes of thrombocytopenia have been methodically ruled out, including:

• central thrombocytopenias, linked to a bone marrow disorder, such as a blood cancer or bone marrow failure;
• thrombocytopenias secondary to an infection, a medication, an autoimmune disease such as lupus, or a liver condition;
• in children, inherited thrombocytopenias, which should be considered in particular before the age of 18 months.

This process of elimination is based on several tests carried out at the time of diagnosis: a full blood count with reticulocyte count and blood smear, ABO and Rh blood grouping with screening for irregular antibodies, particularly in severe forms where a transfusion may be needed, as well as other targeted tests depending on the context.

In both children and adults, disease severity is assessed clinically rather than based only on the platelet count, particularly using the Buchanan score in children, which takes into account the severity of bleeding signs.

A diagnostic journey that can be long but necessary

Because ITP is diagnosed by gradually ruling out other conditions rather than through a positive test, several consultations and additional tests may sometimes be needed before the diagnosis is confirmed.

This period of investigation can feel frustrating for patients living with symptoms without immediate answers, but it remains essential. It helps rule out conditions that would require completely different management and, once the diagnosis is made, guides patients towards the most appropriate treatment.

Key takeaways

Immune thrombocytopenia (ITP) is a diagnosis of exclusion: no biological test can directly confirm it, and the diagnosis is made after other causes of thrombocytopenia have been ruled out.

The diagnosis is considered in the presence of isolated thrombocytopenia, with a platelet count below 100 G/L, confirmed by a second blood sample and associated with an otherwise normal clinical examination.

The condition is sometimes discovered by chance during a blood test or revealed by signs such as bruising, skin purpura, bleeding from the gums or nose, or heavy periods.

The diagnostic journey can take time, as central, secondary, or inherited thrombocytopenias must be methodically ruled out before ITP can be diagnosed.

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