30th April 2024

A 55-year-old woman presents with a one-month history of unexplained weight loss, night sweats, and recurrent infections.

A 55-year-old woman presents with a one-month history of unexplained weight loss, night sweats, and recurrent infections. She mentions feeling more tired than usual and experiencing frequent gum bleeding. She has a past medical history of hypertension, controlled with lisinopril.

Investigations:

Haemoglobin: 100 g/L (120–160)

White cell count: 100 × 10^9/L (4.0–11.0)

Platelets: 60 × 10^9/L (150–400)

Peripheral blood smear: Numerous blast cells

Serum LDH: Elevated

Which initial investigation is most likely to help establish a diagnosis?