Thrombocytopenia: Causes and Workup Topical Map: SEO Clusters

Thrombocytopenia is defined as a platelet count <150 x10^9/L (150,000/µL). Clinically relevant cutoffs are <100 x10^9/L (mild-to-moderate), <50 x10^9/L (increased bleeding risk), and <20–30 x10^9/L (high risk of spontaneous bleeding).

Common causes to evaluate first are drug-induced thrombocytopenia (including heparin), immune thrombocytopenia (ITP), sepsis or critical illness–related consumption, dilutional/platelet loss after massive transfusion, and pseudothrombocytopenia due to EDTA platelet clumping on the CBC. The history, medication list, and peripheral smear usually separate these quickly.

If platelet count is 100–150 x10^9/L without bleeding or risk factors, outpatient recheck in 1–2 weeks and review medications may suffice. Immediate urgent workup (STAT smear, coagulation studies, type & screen, ADAMTS13 if microangiopathy suspected) is warranted for counts <50 x10^9/L, new petechiae, mucosal bleeding, hemolysis, fever, or organ dysfunction.

ITP usually presents as isolated thrombocytopenia without hemolysis or severe organ dysfunction. TTP presents with thrombocytopenia plus microangiopathic hemolytic anemia (schistocytes, elevated LDH, low haptoglobin), neurologic signs, renal dysfunction, and often severe thrombocytopenia; obtain an ADAMTS13 test and start plasma exchange when TTP is strongly suspected.

Bone marrow biopsy is indicated when there's suspicion for marrow failure or infiltration (pancytopenia, abnormal cells), when thrombocytopenia is refractory to standard ITP therapy in older patients, or when peripheral smear suggests dysplasia. It is usually not required for typical new-onset isolated ITP in younger patients.

Severely reduced ADAMTS13 activity (<10%) with an inhibitor is diagnostic for TTP and directs urgent plasma exchange; therefore blood for ADAMTS13 should be drawn immediately and treatment started empirically if clinical suspicion is high since lab turnaround can be 24–72 hours.

Use a 4T pretest probability score first; if intermediate/high, stop heparin and order a PF4/heparin antibody immunoassay and a functional platelet activation assay (SRA or HIPA). Avoid platelet transfusions and use non-heparin anticoagulation if HIT is likely.

Gestational thrombocytopenia (mild, late-pregnancy onset) is common (5–10%) and usually benign; ITP and other causes require specialist input. Management balances maternal bleeding risk and fetal/neonatal thrombocytopenia: treat maternal counts <30 x10^9/L or active bleeding with corticosteroids or IVIG and coordinate delivery planning with obstetrics.

Initial tests: CBC with automated platelet count and review of peripheral smear, reticulocyte count, LDH, haptoglobin, bilirubin, coagulation panel (PT, aPTT), direct antiglobulin test if hemolysis suspected, liver function tests, pregnancy test in women of childbearing age, and targeted serologies/autoimmune panels based on history.

Pseudothrombocytopenia from EDTA-induced platelet clumping shows low automated platelet count but a normal smear will reveal clumps and platelet satellitism; repeat the CBC in a citrate tube or examine a peripheral smear before initiating therapy.