Do we need country-specific blood pressure reference values for children? This question will sound weird for clinicians caring for adult hypertensive patients or researchers working in the domain of adult hypertension. Indeed, there are no country-specific reference values for adults. This contrasts with hypertension in children, for whom there is an increasing number of published sets of country-specific reference values [1–5].
In the current issue of the Journal of Hypertension, Yip et al.  have established 24-h ambulatory blood pressure (ABP) reference values for Hong Kong Chinese children and adolescents. ABP was assessed in a large school-based random sample (89% participation rate) of 1445 participants aged 8–17 years, using a clinically validated oscillometric device  and following standard procedures [1,8]. The authors also investigated the effect on centiles of excluding overweight children and compared them with German reference centiles . ABP was higher in boys than in girls, and increased with age and height. Twenty-one percent of participants were overweight. Compared with the whole sample, the 95th centiles were lower by up to 2.5 mmHg for SBP and by up to 1 mmHg for DBP in normal weight children. The centiles were generally higher than the corresponding German ABP centiles.
Although this study was very well conducted, one question remains unsolved: for which children should we use these reference values? Certainly for Chinese children in Hong Kong. But should they be used for non-Chinese children living in Hong Kong and for Chinese children living in other places in China? And for Chinese children living in Germany or anywhere else in Europe? For the latter, should we not rather use the German reference? In this commentary, I try to address some of the issues at stake and why we need, as in adults, risk-based blood pressure reference values for children.
DISTRIBUTION-BASED BLOOD PRESSURE REFERENCE VALUES IN CHILDREN
Blood pressure is a vital parameter and clinicians need ranges of normal values accounting if necessary for sex, age and height in children , essentially to identify very low and very high levels of blood pressure. Blood pressure is also a risk factor for cardiovascular diseases, and references values are needed to identify children having an increased risk and requiring treatment. Therefore, as in adults, reference should be ideally based on the risk associated with a given level of blood pressure. However, due to the lack of data, reference values are not ‘risk-based’ in children: they are based on the distribution of blood pressure in a reference population, children having elevated blood pressure if their blood pressure is in the upper range of the distribution (Table 1) . For office blood pressure (OBP), the European Society of Hypertension (ESH) recommend the use of US reference, based on data collected in the 1970s and 1980s in 63 227 American children aged 1–17 years [1,10,12].
Using ‘distribution-based’ definition of elevated blood pressure is problematic because the distribution of blood pressure differs between populations. Therefore, the same child can be hypertensive using one reference and normotensive using another reference. For example, because German ABP reference values are lower than Chinese reference values as shown in the study by Yip et al. , a boy having a height of 150 [cm] and a 24-h systolic ABP of 125 [mmHg] would be considered as hypertensive (≥95th centile) using the German reference and normotensive (<95th centile) using the Chinese reference. Should we conclude that a child with this level of ABP in Germany is at a higher risk than a child with the same level of ABP in Hong Kong? There is no evidence to support this assumption. Furthermore, distribution-based reference values are at odds with the possibility that the whole blood pressure distribution may not be optimal for the cardiovascular health of the populations .
MULTIPLICATION OF REFERENCE VALUES
Because the distribution of blood pressure differs from one population to the other, country-specific reference values have been developed. For example, OBP reference values have been published for children from USA , India , Germany , Poland  and Great Britain . Further, because overweight children have higher blood pressure than normal weight children , OBP reference values have been developed for normal weight children from USA  and Germany .
In the current issue of the Journal of Hypertension, Yip et al. have produced ABP reference values for Chinese normal weight children . Implicitly, the idea is to build reference values from ‘healthy’ children. This is a ‘prescriptive-based’ method to establish optimal reference values (Table 1) . The new growth curves of the WHO for infant and young children have been constructed following this principle and are based on data from healthy children who were breastfed and lived in good hygiene conditions ; they constitute a growth standard . A blood pressure standard would require data on children not only with a normal weight but also having a healthy diet, being regularly physically active, living in favourable socio-economic situations and having normotensive parents.
Some authors have also argued in favour of measurement method-specific reference values, that is, different reference values for oscillometric and auscultatory measurement method [4,18]. If clinically validated oscillometric devices are used, there is no reason to develop specific reference values for oscillometric devices . Nevertheless, it is necessary to have different reference values for blood pressure in the office and for blood pressure out of the office.
RISK-BASED BLOOD PRESSURE REFERENCE VALUES IN CHILDREN
To avoid several problems of distribution-based reference values, risk-based reference values are needed, like in adults . One method would be to develop reference on the basis of the association between blood pressure level in children and intermediate outcomes, such as left ventricular mass or intima–media thickness . However, it requires outcomes and blood pressure data on a large number of children and adolescents.
Another method would be to derive childhood centile curves passing at age 18 through the adult cut-off of 140/90 mmHg to define hypertension. This (distribution and risk-based) method was used successfully for the international definition of childhood overweight and obesity in children using body mass index . One major interest of this method is the continuity of blood pressure categories from childhood through adulthood, by the explicit link between hypertension in childhood and hypertension and associated risk in adulthood.
Rather than multiple country-specific reference values, establishing one universal set of reference values for OBP and one for ABP – both risk-based – should be seriously considered.
No funding was received for this study.
Conflicts of interest
There are no conflicts of interest.
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