What initial investigations should I perform for a patient presenting

Guideline-aligned answer with reasoning, red flags and references. Clinically reviewed by Dr Kola Tytler MBBS CertHE MBA MRCGP.

Initial Investigations for Thrombocytopenia

The primary initial investigation for a patient presenting with thrombocytopenia is a full blood count (FBC) ,,,. This should be considered very urgent (within 48 hours) if there is unexplained bleeding, bruising, or petechiae .
A comprehensive assessment should be conducted to determine the possible underlying cause of the thrombocytopenia . This includes reviewing the patient's medical history and current medications, as some medicines can be associated with thrombocytopenia .
If a bleeding disorder is suspected, or if there is abnormal bruising, an abnormal clotting screen may be considered as part of further investigations . For women with heavy menstrual bleeding, testing for coagulation disorders (e.g., von Willebrand's disease) should be considered if they have had heavy menstrual bleeding since their periods started and have a personal or family history suggesting a coagulation disorder .
Depending on the initial platelet count and clinical context, a repeat full blood count may be arranged . For instance, if the platelet count is 50–100 × 10^9/L without other referral criteria, a repeat FBC in 1–2 weeks may be appropriate . If a platelet count is 100–150 × 10^9/L with no underlying cause identified, a repeat FBC in 4–6 weeks is suggested . If a medicine is suspected to be the cause, the count should be reviewed and repeated in 1–2 weeks after considering stopping the medicine .

What initial investigations should I perform for a patient presenting with thrombocytopenia?