Laboratory reasoning

FBC and blood film — pattern recognition

Overview and approach

Interpret haemoglobin, cells and indices together; blood-film findings may identify urgent pathology.

  • Look at the film when counts are abnormal; a film is vital to exclude clots and to look for blasts or dysplasia (1).
Assessment and clinical features

Symptoms, bleeding, infection, medicines, nutrition, ethnicity and prior counts.

  • Thrombocytopenia: review drugs started in the previous 10 days (quinine, trimethoprim–sulfamethoxazole, vancomycin, heparin and chemotherapy are common culprits); consider liver disease, splenomegaly, B12 or folate deficiency, HIV, hepatitis C, recent infection or live vaccine, autoimmune disease, pregnancy (1).
  • Neutropenia: usually transient, related to infection or drugs; review drugs and alcohol; ask about sepsis symptoms (2).
Differentials and investigations

Reticulocytes, film and targeted haemolysis/nutritional studies; several cytopenias suggest a broader process.

  • First-line for thrombocytopenia: FBC and film, fibrinogen, D-dimer, clotting, haemolysis screen, B12 and folate, LFTs, HIV and hepatitis serology, renal function (1). Isolated neutropenia referral information: serial FBC, E/LFTs, drug and alcohol history; flow cytometry and autoimmune screen if requested (2).
Management and follow-up

Treat cause and seek haematology advice for unexplained or severe abnormalities; transfusion decisions are clinical, not a number alone.

  • Asymptomatic platelets 50 to 100 × 10^9/L with no other abnormality: repeat in 1 to 2 weeks; 100 to 150: repeat in a month or more (1). Cease a suspected causative drug before referring (1).
  • Isolated neutropenia, no sepsis: below 0.5 × 10^9/L category 1 outpatient referral; 0.5 to 1.0 category 2; 1.0 to 1.5 category 3 (2).
Red flags and clinical pearls

Blasts, severe neutropenia with fever, major thrombocytopenia/bleeding or microangiopathic haemolysis needs urgent review.

  • Emergency referral: neutropenia below 1.0 × 10^9/L with sepsis signs, circulating blasts, abnormal lymphoid cells, low fibrinogen, pancytopenia (2). Thrombocytopenia with bleeding, blasts, haemolysis with neurological or renal features (TTP/HUS), heparin exposure with skin necrosis (HIT: stop heparin), or DIC (1).
References
  1. RACGP (AFP): Incidentally detected thrombocytopaenia in adults (2014)
  2. Queensland Health CPC: Neutropenia (isolated)

Last updated from Sen’s notes on 9 October 2026.