What labs should be checked in a patient with thrombocytopenia (platelet count of 100) and a postinfectious cough?

Laboratory Workup for Thrombocytopenia (Platelet Count 100,000/μL)

For a patient with a platelet count of 100,000/μL and postinfectious cough, begin with a peripheral blood smear to exclude pseudothrombocytopenia, followed by a CBC with differential, reticulocyte count, and targeted infectious/autoimmune serologies based on clinical context.

Initial Essential Laboratory Tests

First-Line Tests (Obtain Immediately)

• Peripheral blood smear examination is mandatory to exclude pseudothrombocytopenia (EDTA-induced platelet clumping), identify platelet satellitism, detect large/giant platelets, and assess for other cell line abnormalities 1
• Complete blood count (CBC) with differential to evaluate for concurrent cytopenias (anemia, leukopenia) that would suggest bone marrow pathology or Evans syndrome 2
• Reticulocyte count to assess for concurrent hemolysis, which when present with thrombocytopenia suggests Evans syndrome 2

Critical pitfall: Pseudothrombocytopenia occurs in up to 0.1% of samples and is caused by EDTA-induced platelet agglutination. If suspected on smear, immediately redraw blood in sodium citrate or heparin tubes to obtain accurate platelet count 1. This is a benign laboratory artifact requiring no treatment.

Second-Line Tests Based on Clinical Context

Given the postinfectious presentation, prioritize:

• Viral serologies: HIV, hepatitis C virus (HCV), hepatitis B virus (HBV), and CMV testing, as these infections commonly cause thrombocytopenia and are specifically recommended in the ITP workup 2
• Direct antiglobulin test (DAT/Coombs) to rule out concurrent autoimmune hemolytic anemia (Evans syndrome), particularly if reticulocyte count is elevated 2
• Prothrombin time (PT) ratio (not INR alone, as subtle changes are missed with INR reporting) and fibrinogen level if any coagulopathy is suspected 2

Additional Testing for Specific Clinical Scenarios

If Immune Thrombocytopenia (ITP) Suspected

• H. pylori testing (stool antigen or urea breath test) is recommended for all newly diagnosed ITP patients 2
• Nutritional assessment including vitamin B12, folate levels if dietary deficiency suspected 2
• Antinuclear antibody (ANA) and antiphospholipid antibodies if systemic lupus erythematosus or antiphospholipid syndrome suspected 3, 4

If Bone Marrow Pathology Suspected

Bone marrow evaluation is NOT routinely indicated for isolated thrombocytopenia at 100,000/μL. Reserve bone marrow biopsy only if: 2

• Abnormalities in other cell lines (anemia, leukopenia) are present
• Peripheral smear shows atypical cells
• Patient fails to respond to initial management
• Concern for aplastic anemia or myelodysplastic syndrome exists

If Postinfectious/Viral Etiology Suspected

Given the postinfectious cough context:

• Reticulated platelet percentage can help differentiate increased platelet destruction (>11% suggests peripheral destruction like ITP) from decreased production (≤11% suggests bone marrow issue), with 93% sensitivity and 85% specificity 5
• Chest X-ray to evaluate for thymoma if autoimmune etiology suspected 2

Risk Stratification Based on Platelet Count

At 100,000/μL (Grade 1 thrombocytopenia): 2

• Patients are generally asymptomatic with minimal bleeding risk 6
• Continue monitoring with close clinical follow-up
• No immediate intervention required unless bleeding occurs
• Repeat platelet count in 1-2 weeks to establish trend

Important context: The American Society of Hematology defines ITP using a threshold of <100,000/μL, recognizing that counts between 100-150,000/μL may be normal variants in certain populations and have only 6.9% chance of progressing to persistent thrombocytopenia over 10 years 2

What NOT to Order Routinely

• Bone marrow biopsy is not indicated for isolated mild thrombocytopenia without other cytopenias 2
• Antiplatelet antibody testing has poor sensitivity/specificity and is not recommended for ITP diagnosis 2
• Extensive thrombophilia workup unless clinical thrombosis present (thrombocytopenia alone does not indicate thrombophilia testing)

Monitoring Strategy

• Repeat CBC weekly if platelet count is stable at 100,000/μL and patient asymptomatic 2
• More frequent monitoring (every 2-3 days) if platelet count trending downward or if patient develops bleeding symptoms 7
• D-dimer, PT, and fibrinogen should be added to monitoring if platelet count drops below 50,000/μL or if coagulopathy develops 2