A 50-year-old woman is found to have oesophageal varices at endoscopy performed because of dyspepsia. She is not aware of any signs and symptoms of previous liver diseases and has no medication. Serum liver tests are normal. Splenomegaly and portal vein thrombosis with cavernoma formation are found at abdominal ultrasonography and confirmed at abdominal CT where no evidence for intra-abdominal inflammatory processes or malignancy is found. Elastography of the liver is normal.

Which of the following diagnostic steps is first indicated?