14 Chapter 1 individual at home (point-of-care testing). In fact, it has been shown that for a substantial part of the commonly used drugs, the summary of product characteristics (SmPC) (i.e., the drug label) or the clinical guidelines advise (“prescribe”) the measurement of laboratory markers . The aim of drug dosing is to reach a so-called therapeutic window. The balance between benefit and risk (i.e., desired effect and toxicity) is considered optimal for an individual in this window  (see Figure 1). Subtherapeutic effects are expected under the window and supratherapeutic due to maximum effect resulting in toxic or financial toxicity. For some drugs (e.g., aminoglycosides, lithium, digoxin), the therapeutic window is narrow, and strict monitoring is of the essence. Monitoring can involve the measurement of either an endogenic biomarker (e.g., glucose, INR, anti-Xa) or the measurement of the drug in plasma, serum, blood or other specimens. The latter is called therapeutic drug monitoring (TDM). Figure 1. Dose-response curve with a therapeutic window. Monitoring of FVIII The standard monitoring approach in haemophilia management does not involve the measurement of a drug concentration; instead, the biomarker ‘FVIII activity’ is used. This FVIII activity is monitored to diagnose haemophilia A, to dose the FVIII products and to provide clinical support to PwHA (e.g., during surgery or bleeding) (see Figure 2). FVIII activity is used for diagnosing purposes to classify the severity of haemophilia A. The frequency of monitoring and treatment strategy are based on the severity classification.