Monitoring entails periodic measurement to guide management1 and is widely practised in clinical medicine to inform decisions throughout the course of a disease and to provide prognostic information to patients. It is helpful to divide monitoring into phases: pretreatment, initial response, maintenance, re-establish control, and post-treatment.1

Initial response monitoring uses repeat measurement soon after a new treatment is started to check that the response is within a range that maximises the benefits while minimising the harms. Table 1 summarises different types of initial response monitoring.⇓ We have limited our discussion to the use of surrogate outcomes for monitoring initial response to treatment. Surrogate outcomes are commonly used to monitor initial response in patients with chronic conditions. This type of initial response monitoring is common in clinical practice and can result in inappropriate decisions. We looked at two scenarios to develop a rational framework for deciding whether this form of initial monitoring should be done: Should change in blood pressure be monitored after addition of a diuretic to an angiotensin II receptor blocker in adults with essential hypertension? and Should change in cholesterol be monitored after giving patients with ischaemic heart disease a statin?