A fundamental property of blood pressure (BP) is its variability, which is essential for adaptation to numerous changes in the prevailing circumstances. Thus, blood pressures measured at the doctors’ offices or clinics are the results of the adaptive responses to, for example, the anxieties and expectations of the patients in those special situations and environments. These BPs do not represent the BPs measured outside the clinic at various time points during the days and the nights. In this respect, ambulatory blood pressure monitoring (ABPM) over a 24-h or 48-h period provides more representative information than BP measurements in the office. However, ABPM is expensive, technically more difficult, not available in many of the developing countries, and not widely available even in many centers in the developed countries.

If available, clinicians should use ABPM generously for the diagnosis of, and the follow up of the treatment of hypertension (O’Brien 2016). The technique should be made available more widely both in the primary care centers as well as in the hospital clinics. If the ABPM services do not exist, then the clinicians should take initiatives, and influence the decision makers to make the technique available to the patients at affordable costs.

In this review, I shall address some selected issues for a better understanding of the rationale, and the utility of ABPM, in an attempt to persuade clinicians to use the technique more liberally in their practices. I shall also touch upon some practical issues, which may help clinicians feel comfortable in using the technique, interpreting the results, and making treatment decisions more correctly. For more comprehensive information on ABPM, clinicians may read some recent reviews and guidelines (O’Brien et al. 2013; Parati et al. 2014; Hermida et al. 2013a).

Even when freely available, use of ABPM for all patients will not be cost effective. Physicians need to make decisions about the use of ABPM in individual cases based on sound clinical judgment and experience, even in the absence of convincing evidence, or clear guidelines. In principle, it should be used to identify eventual discrepancies between the office blood pressure measurement (OBPM) and the ABPM. ABPM should be performed, in selected cases, preferably for two consecutive 24 h periods, for confirming the diagnosis of different forms of hypertension, for evaluating the severity of the condition during a 24-h period, for diagnosing sleep-time hypertension, for identifying the dipping patterns, episodes of hypertension or hypotension, and for identifying the patients with autonomic failures (O’Brien et al. 2013). In the following paragraphs, I shall briefly describe some of these conditions.

Some patients, who are not on any anti-hypertensive medicines, have high blood pressure (BP) on repeated measurements (usually day time awake BP) at the doctors’ office (OBPM). If ABPM is not available, most doctors will label these patients as hypertensive patients, and they will do so rightly in about 80 % of the cases. However, they will be wrong in about 20 % of the cases, who actually have normal BP (as evidenced from ABPM), but are wrongly diagnosed as hypertensive patients and treated unnecessarily by antihypertensive medicines, often for the rest of their lives. This is a serious problem for the patients and is expensive for the society, and it must be avoided.

Men who have high office BP (>140/90) but whose mean 24-h BP is normal (i.e. ≤130/80 mmHg), do not have hypertension, and should not be treated with antihypertensive drugs (O’Brien et al. 2013). These people have “masked normotension” (also called “white-coat hypertension”, or “isolated office- hypertension”), and identification of these people by ABPM is important. They cannot be identified by home blood pressure monitoring (HBPM), which cannot measure sleep-time BP, an important marker of cardiovascular disease (CVD) risk. These people should be followed up by ABPM within two years, if they do not have an increased risk for CVD (e.g. diabetes, chronic kidney disease, or past CVD), or within one year, if they have an increased CVD risk.

In the USA, the Centers for Medicare and Medicaid Services approves reimbursement for the use of ABPM for identification of people with “masked normotension”. Early use of ABPM for the diagnosis of “masked normotension” is recommended, among others, by the Canadian Hypertension Education Program (CHEF), UK National Institute for Health and Clinical Excellence (NICE), the European Society of Hypertension, and the International Society for Chronobiology (O’Brien et al. 2013; Hermida et al. 2013a; Gelfer et al. 2015; Krause et al. 2011). U.S. Preventive Services Task Force (USPSTF) concludes that ABPM is the reference standard for confirming elevated office BP results to avoid misdiagnosis and overtreatment of persons with isolated office-hypertension (Piper et al. 2015). ABPM before starting treatment of hypertension is cost-effective (Lovibond et al. 2011).

It should be noted that if office BP is ≥180/110, doctors need to start treatment without waiting for ABPM. First-time ABPM should not be done for patients who are on ≤2 anti-hypertensive medicines. If they are on ≥3 anti-hypertensive medicines then they should be tested by ABPM to confirm if they have true resistant hypertension (see below).

Many people, who are not on any anti-hypertensive medicines, have normal office BP measured on repeated occasions. It is important for clinicians to keep in mind that about 10 % of these people may have hypertension, despite normal office BP (“masked hypertension”). In some cases “masked hypertension” is due to sleep-time hypertension. “Masked hypertension” is common and is associated with increased risk for CVD events (Hermida et al. 2012; Booth et al. 2016). If the diagnosis of hypertension is missed in people with “masked hypertension”, they will remain untreated with increased risk for CVD. For the diagnosis of “masked hypertension”, clinicians should use ABPM whenever they suspect the condition.

Sleep-time hypertension and the “dipping” or “rising” patterns of BP during sleep-time can be identified only by using 24-h ABPM (preferably on two consecutive days). It should be noted that for defining the wake-time BP and the sleep time BP, it is common and convenient to use arbitrary fixed clock hours, for example, wake-time BP defined as BP measured during 09:00–21:00, and sleep-time BP defined as those during 01:00–06:00. However, presentation of ABPM results in terms of clock hours can be misleading. For accurately measuring the average wake-time and sleep-time BPs, people undergoing ABPM must note down in a diary the times of retiring to the bed at night, and the times of awakening in the morning, and the ABPM results must be presented in terms of “hours from bedtime” (Hermida et al. 2013a).

Some people have normal office BP, and normal ABPM-derived mean wake-time BP, but high ABPM-derived mean sleep-time BP. The latter is an independent, and a better predictor of CVD morbidity and mortality compared to the office BP or ABPM-derived mean wake-time BP, or mean 24 h BP (Hermida et al. 2016). Sleep-time relative systolic BP (SBP) decline has additional prognostic value. Sleep time relative SBP decline or “dipping” is calculated as 100× (mean wake-time SBP − mean sleep-time SBP)/mean wake-time SBP. Sleep-time relative SBP decline is a continuous variable but it is conventional to divide people into four groups based on the decline: people who have >10 % decline in sleep-time SBP compared to the wake-time SBP are called “dippers”; those who have <10 % decline are called “non-dippers”; those who have >20 % decline are called “extreme dippers”; and those who have <0 % decline are called “risers”. It should be noted that in shift-workers, the circadian rhythm of BP is reversed. They have peak BP at about 10:00–11:00 (Sternberg et al. 1995).

The frequency of sleep-time hypertension, “non-dipping”, “reduced-dipping” or “riser” patterns of BP is high (as high as 65–81 %) in the elderly people, type 2 diabetes, chronic kidney disease, obstructive sleep apnea and other sleep disorders, resistant hypertension, obesity and pregnancy (Hermida et al. 2016; Mojon et al. 2013; Ayala et al. 2013). Moreover, about 20 % “normotensive” people are “non-dippers”, and thus have increased CVD risk. ABPM should be performed when sleep-time hypertension, “reduced dipping”, “non-dipping” or “rising” patterns of sleep-time BP are suspected. Identification of sleep-time hypertension and the “non-dippers” is clinically useful, since some of these patients can be treated by administering some of the antihypertensive medicines, in full dose, at the bed-time (not traditional BID regimen) (Hermida et al. 2016).

Hypertension that is not controlled by lifestyle changes and therapeutic doses of ≥3 antihypertensive medicines (including a diuretic, unless contraindicated) is called resistant hypertension. Thus, all patients who need ≥4 medicines for control of BP have resistant hypertension. Diagnosis of resistant hypertension based on OBPM can often be wrong because of the well-known “white-coat effect”. In one large study, 37.5 % of the “resistant hypertension” patients diagnosed by OBPM had “white-coat resistant hypertension”, and 62.5 % had true resistant hypertension, as verified by ABPM (de la Sierra et al. 2011). ABPM is essential for a correct diagnosis of resistant hypertension (Lazaridis et al. 2015). It is also useful for guiding the treatment of hypertension in these patients. When any treatment of these patients is modified in any way, the results of such modifications should be evaluated by repeating ABPM within the ensuing three months.