Laboratory Workup for Petechiae
Order a complete blood count (CBC) with peripheral blood smear, prothrombin time (PT), and activated partial thromboplastin time (PTT) as initial hemostasis tests for any patient presenting with petechiae. 1, 2
Initial Laboratory Panel
The foundational laboratory evaluation should include:
- Complete blood count (CBC) - Essential to identify thrombocytopenia, leukopenia, or other cytopenias that may explain the petechiae 1, 2
- Peripheral blood smear - Critical for identifying platelet morphology abnormalities, immature bands, or other cellular abnormalities 1
- Prothrombin time (PT) - Evaluates for coagulation factor deficiencies 1
- Activated partial thromboplastin time (PTT) - Assesses intrinsic coagulation pathway 1
These initial tests do not evaluate for von Willebrand disease specifically, but they identify whether coagulation factor deficiency or thrombocytopenia might be causing the clinical bleeding manifestations 1.
Additional Testing Based on Clinical Context
If Thrombocytopenia is Confirmed
- HIV and hepatitis C (HCV) testing - Recommended for all adults with newly diagnosed immune thrombocytopenia (ITP), as treatment of underlying infection may alter the course of secondary ITP 1, 2
- Comprehensive metabolic panel - Particularly useful to assess for liver disease (which can cause thrombocytopenia), hyponatremia, and hepatic transaminase elevations 1
- Blood type and cross-match - Should be ordered if there are signs of severe bleeding 1
If Bleeding History is Strong
Consider adding von Willebrand disease (VWD) screening tests 1:
- VWF antigen (VWF:Ag)
- VWF ristocetin cofactor activity (VWF:RCo)
- Factor VIII coagulant activity (FVIII)
All three VWD tests should be ordered together, as results may establish both diagnosis and suggest the type and severity of VWD if present 1.
Critical Clinical Considerations
Exclude pseudothrombocytopenia first - If isolated thrombocytopenia is found in a stable outpatient, collect blood in a tube containing heparin or sodium citrate and repeat the platelet count to rule out laboratory artifact 3.
Assess for emergency conditions - In ill-appearing patients with fever and petechiae, do not delay parenteral antibiotics while awaiting laboratory results, as meningococcal disease requires immediate treatment 1, 2. Look specifically for:
- Fever or history of fever 2
- Hepatosplenomegaly or lymphadenopathy 2
- Neck stiffness or meningeal signs 2
Review the peripheral smear carefully - Large hypogranular platelets may indicate inherited platelet disorders like gray platelet syndrome that can be masked by concurrent ITP 4. The presence of immature bands suggests infection, while leukopenia with thrombocytopenia is particularly suggestive of ehrlichiosis or anaplasmosis 1.
What NOT to Order Routinely
- Bone marrow examination - Not necessary in typical ITP presentations regardless of age 1, 2
- Antinuclear antibodies - Not necessary in initial evaluation of suspected ITP 1
- H. pylori testing - Not recommended routinely in children with chronic ITP 1
- Antiplatelet or antiphospholipid antibodies - Insufficient evidence to recommend routine use in suspected ITP 1
Common Pitfalls
Do not assume all petechiae require extensive workup - Well-appearing infants with localized petechiae (especially to lower limbs) often have benign etiologies like tourniquet phenomenon, and may only require observation rather than full laboratory investigation 2, 5.
Do not overlook medication history - Drug-induced thrombocytopenia is a common cause and should be assessed through careful medication review 2, 3.
Do not miss abnormalities beyond thrombocytopenia - If the CBC shows abnormalities other than isolated thrombocytopenia (such as leukopenia, anemia not attributed to blood loss, or abnormal white cell morphology), further investigation including possible bone marrow examination is warranted before diagnosing ITP 1.