What laboratory tests should be ordered in the initial work‑up of a patient with thrombocytopenia?

Laboratory Work-Up of Thrombocytopenia

Initial Essential Tests

The cornerstone of thrombocytopenia evaluation is a complete blood count with differential and manual peripheral blood smear review by a hematopathologist—this single step excludes pseudothrombocytopenia, identifies isolated versus multi-lineage cytopenias, and reveals morphologic clues that direct all subsequent testing. 1, 2

Step 1: Confirm True Thrombocytopenia

• Repeat platelet count in citrate or heparin tube if EDTA-dependent platelet clumping is suspected; pseudothrombocytopenia occurs in approximately 0.1% of samples and must be excluded before any further work-up 1, 2, 3
• Manual smear review is mandatory—automated counters miss pseudothrombocytopenia, giant platelets, schistocytes, and leukocyte abnormalities that carry life-threatening implications 1, 2

Step 2: Complete Blood Count Analysis

• Verify isolated thrombocytopenia (platelet count <100 × 10⁹/L with normal hemoglobin and white blood cell count); any additional unexplained cytopenias mandate immediate bone marrow examination 1, 2
• Reticulocyte count helps distinguish bleeding-related anemia from marrow failure 2

Step 3: Peripheral Blood Smear Findings

Expected findings in primary immune thrombocytopenia (ITP):

• Platelets normal-sized or mildly enlarged (not giant) 1, 2
• Normal red blood cell morphology without schistocytes 1, 2
• Normal white blood cell morphology without immature cells or inclusion bodies 1, 2

Red flags requiring alternative diagnosis:

• Schistocytes → thrombotic microangiopathy (TTP/HUS/DIC); untreated TTP carries >90% mortality—obtain ADAMTS13 activity, LDH, haptoglobin, coagulation studies urgently 1, 2
• Giant platelets (approaching red cell size) → inherited thrombocytopenias (MYH9-related disease, Bernard-Soulier syndrome) 1, 2
• Leukocyte inclusion bodies → MYH9-related disease 1, 2
• Immature or abnormal white cells → leukemia or myelodysplastic syndrome 1, 2

Mandatory Infectious Disease Testing (All Adults)

These tests must be performed in every adult with suspected ITP, regardless of risk factors, because these infections can masquerade as primary ITP for years:

• HIV antibody testing—HIV-associated thrombocytopenia is clinically indistinguishable from primary ITP and may precede other HIV manifestations 1, 2
• Hepatitis C virus serology—chronic HCV causes secondary ITP that resolves with antiviral therapy 1, 2
• Helicobacter pylori testing (urea-breath test or stool antigen preferred over serology)—eradication normalizes platelet counts in a subset of adult ITP patients, especially in high-prevalence regions 1, 2

Additional Context-Specific Laboratory Tests

Clinical Scenario	Required Test	Rationale
Any heparin exposure in past 3 months	Calculate 4T score immediately; if ≥4, obtain anti-PF4 antibodies and stop all heparin [2,4]	HIT carries 30-50% thrombosis risk; delay in diagnosis is fatal [2]
Suspected thrombotic microangiopathy (schistocytes present)	ADAMTS13 activity, LDH, haptoglobin, PT/aPTT, fibrinogen, D-dimer [2,5]	Untreated TTP has >90% mortality; treatment must begin before confirmation [2]
Women of childbearing potential	Pregnancy test [1,2]	Differentiates gestational thrombocytopenia, pre-eclampsia, HELLP syndrome [2]
Suspected Evans syndrome	Direct antiglobulin test (DAT) [2]	Identifies concurrent autoimmune hemolytic anemia [2]
Before anti-D immunoglobulin therapy	Rh(D) blood group typing [1,2]	Anti-D only effective in Rh(D)-positive patients [2]
Suspected common variable immune deficiency	Quantitative immunoglobulin levels [1,2]	ITP can be presenting feature of CVID [2]

Coagulation Studies (When Indicated)

Obtain PT, aPTT, fibrinogen, and D-dimers if:

• Platelet count <20 × 10⁹/L with active bleeding 2
• Clinical suspicion for disseminated intravascular coagulation (DIC) 2, 5
• Schistocytes present on smear 2

Indications for Bone Marrow Examination

Bone marrow aspiration and biopsy (with flow cytometry and cytogenetics) are mandatory when ANY of the following are present:

• Age ≥60 years—to exclude myelodysplastic syndrome, leukemia, or other malignancies 1, 2
• Systemic constitutional symptoms (fever, unexplained weight loss, night sweats, bone pain) 1, 2
• Abnormal CBC parameters beyond isolated thrombocytopenia (unexplained anemia, leukopenia, leukocytosis) 1, 2
• Atypical peripheral smear findings (schistocytes, immature cells, giant platelets, leukocyte inclusion bodies) 1, 2
• Splenomegaly, hepatomegaly, or lymphadenopathy on physical exam—these findings exclude primary ITP 1, 2
• Minimal or no response to first-line ITP therapies (IVIg, corticosteroids, anti-D) 1, 2
• Prior to splenectomy in chronic ITP 1, 2

Bone marrow examination is NOT required in patients with typical ITP features: isolated thrombocytopenia, age <60 years, normal physical exam except for bleeding manifestations, normal smear morphology, and no systemic symptoms 1, 2

Tests That Should NOT Be Ordered Routinely

The following tests lack diagnostic specificity or proven clinical utility and should not be performed routinely:

• Platelet-associated IgG (PaIgG) 1, 2
• Glycoprotein-specific antiplatelet antibodies 1, 2
• Thrombopoietin (TPO) levels 1, 2
• Reticulated platelet counts 1, 2
• Bleeding time 1, 2
• Antiphospholipid antibodies—only if clinical features of antiphospholipid syndrome are present 1, 2
• Antinuclear antibodies (ANA)—only when systemic lupus erythematosus is suspected 1, 2

Critical Diagnostic Pitfalls

Never diagnose ITP without personal hematopathologist review of the peripheral smear—automated counters miss pseudothrombocytopenia, giant platelets, and schistocytes 1, 2

Never omit HIV and HCV testing in adults, even in low-risk populations—these infections can be clinically indistinguishable from primary ITP and may precede other symptoms by years 1, 2

Missing TTP, HIT, or DIC carries catastrophic mortality risk (untreated TTP >90% mortality; HIT thrombosis risk 30-50%)—prompt exclusion of these entities through smear review, 4T score calculation, and coagulation studies is essential 2, 4, 5

Presence of splenomegaly, hepatomegaly, or lymphadenopathy excludes primary ITP—these findings mandate investigation for HIV, systemic lupus erythematosus, lymphoproliferative disorders, or chronic liver disease 1, 2