Invasive aspergillosis (IA) is the most common mould infection in immunocompromised patients with haematological disease. Voriconazole, a triazole, improves overall survival of patients with an IA and is the mainstay of therapy. Resistance of A. Fumigatus emerged as an important clinical problem and infections with azole resistant Aspergillus have a high mortality. Nowhere in the world, azole resistance is more prevalent than in the Netherlands. Rapid detection of resistance is key to improve the patient's outcome but fungal cultures take time and are often negative. The investigators aim to detect azole resistance associated mutations in fungal DNA extracted directly from serum or plasma to accelerate diagnosis and improve outcome of patients infected with azole resistant A. fumigatus.