What are the possible causes of thrombocytopenia in a healthy teenager?

Causes of Low Platelet Count in a Healthy Teenager

In a healthy teenager with thrombocytopenia, primary immune thrombocytopenia (ITP) is the most likely diagnosis, typically presenting as isolated thrombocytopenia following a viral infection, with the majority of cases resolving spontaneously within 6 months. 1, 2

Most Common Causes in This Population

Primary Immune Thrombocytopenia (ITP)

• ITP is the most frequent cause of sudden-onset thrombocytopenia in otherwise healthy children and adolescents, accounting for the majority of cases in this age group 3, 4
• Typically occurs between ages 1-7 years but can present during teenage years, often 1-4 weeks following a viral infection 3, 5
• Characterized by isolated thrombocytopenia with normal hemoglobin, white blood cell count, and peripheral blood smear (except for low platelets that may appear larger than normal) 1, 2
• Approximately two-thirds of children improve spontaneously within days to 6 months, with complete remission occurring in 70-80% of cases 1, 3
• Only 20-30% develop persistent thrombocytopenia beyond 6 months 3

Post-Viral or Post-Vaccination Thrombocytopenia

• Sudden severe thrombocytopenia can occur after viral infections including Epstein-Barr virus, cytomegalovirus, varicella zoster virus, or following vaccinations (measles-mumps-rubella) 5, 6
• This mechanism likely involves molecular mimicry where antibodies generated against viral antigens cross-react with platelet surface antigens 5, 6
• Clinically indistinguishable from acute ITP and managed similarly 5

Secondary Causes to Exclude

Infection-Related Thrombocytopenia

• HIV, hepatitis C, and H. pylori should be tested in adolescents with suspected ITP, as these infections can cause secondary immune thrombocytopenia 2, 7, 5
• Viral infections can suppress bone marrow platelet production directly 2
• Testing is particularly important before initiating immunosuppressive therapy 2, 7

Drug-Induced Thrombocytopenia

• Obtain detailed medication history including over-the-counter drugs, herbal supplements, and recent antibiotic use 2, 7, 8
• Common culprits include antibiotics, NSAIDs, anticonvulsants, and heparin products 8, 5

Autoimmune Disorders

• Secondary ITP can occur with systemic lupus erythematosus, antiphospholipid syndrome, or autoimmune thyroid disease 2, 7, 5
• Consider testing antinuclear antibodies (ANA) and antiphospholipid antibodies if clinical features suggest underlying autoimmune disease 1, 2

Inherited Thrombocytopenias

• Family history and examination of platelet size on peripheral smear provide critical clues 2, 7
• Conditions include MYH9-related disease, Wiskott-Aldrich syndrome, and 22q11.2 deletion syndrome 2, 7
• These typically present with lifelong mild-to-moderate thrombocytopenia rather than acute onset 2

Essential Diagnostic Approach

Initial Laboratory Evaluation

• Complete blood count with differential to confirm isolated thrombocytopenia versus pancytopenia 2, 7, 8
• Peripheral blood smear examination by qualified personnel to exclude pseudothrombocytopenia (EDTA-dependent platelet clumping) and identify abnormal cells 1, 2, 7
• If pseudothrombocytopenia suspected, repeat platelet count using heparin or sodium citrate tube 8

Physical Examination Red Flags

• Physical examination should be normal except for bleeding manifestations (petechiae, purpura, ecchymosis) 1, 2
• Presence of splenomegaly, hepatomegaly, or lymphadenopathy strongly suggests alternative diagnoses such as lymphoproliferative disorders or storage diseases 1, 2
• Non-petechial rash, fever, weight loss, or bone pain indicate need for bone marrow examination 2

When Bone Marrow Examination Is NOT Needed

• Bone marrow examination is not necessary in typical ITP presentation: isolated thrombocytopenia, normal physical exam except bleeding signs, normal CBC parameters except platelets, and age under 60 years 1, 2, 7

When Bone Marrow Examination IS Required

• Age over 60 years (not applicable to teenagers) 2, 7
• Systemic symptoms present (fever, weight loss, bone pain) 2, 7
• Abnormal blood count beyond thrombocytopenia (anemia, leukopenia, abnormal white cell morphology) 1, 2, 7
• Atypical peripheral smear findings (schistocytes, immature cells, giant platelets approaching red cell size) 1, 2
• Failure to respond to first-line ITP therapies 2, 7

Clinical Significance and Risk Assessment

Bleeding Risk Stratification

• Severe bleeding occurs in only 3% of children with ITP 1
• Bleeding risk increases significantly when platelet count falls below 10 × 10⁹/L 1, 8
• Intracranial hemorrhage risk is approximately 0.1-0.5% in children with ITP, with most occurring within first 5 weeks of diagnosis 1
• Risk factors for serious bleeding include head trauma and concurrent use of medications affecting platelet function (aspirin, NSAIDs) 1

Lifestyle Considerations for Teenagers

• During teenage years, lifestyle and self-image issues assume greater importance and should influence management decisions 1
• Adolescents should avoid competitive contact sports with high risk of head trauma 1
• For adolescent girls, menstruation may be problematic and can be managed with antifibrinolytic agents and hormonal contraceptives 1

Common Diagnostic Pitfalls to Avoid

• Failing to exclude pseudothrombocytopenia by not examining peripheral smear or repeating count in different anticoagulant 2, 8
• Missing secondary causes (HIV, HCV, H. pylori, autoimmune diseases) that have different natural histories and treatment responses 2, 7, 5
• Not recognizing inherited thrombocytopenias that may present similarly but require different management 2, 7
• Overlooking drug-induced thrombocytopenia by not obtaining comprehensive medication history including supplements 2, 7, 8
• Assuming elevated immature platelet fraction definitively confirms ITP, as severe ITP can present with low immature platelet fraction 2
• Missing constitutional symptoms (fever, weight loss) that suggest lymphoproliferative disorders or bone marrow failure rather than primary ITP 2, 7