CARDIOVASCULAR JOURNAL OF AFRICA • Vol 22, No 4, July/August 2011
210
AFRICA
are shown in Table 5. The optimal WHtR cut-off value with the
highest sum of sensitivity and specificity values was the same
in both boys and girls, namely 0.41. For this reason, diagnostic
accuracy measures were computed for 0.41 and 0.5 WHtR cut-
off values.
The level of sensitivity of a WHtR cut-off point of 0.5 was
very low for both boys and girls (4.8 and 7.6%, respectively).
The cut-off value of 0.41, which corresponds to a sensitivity of
61.9% and a specificity of 53.6% in boys seems to be the most
appropriate and the same cut-off value was found for girls, but
with a sensitivity of 62% and a specificity of 51%. The fact that
the cut-off value obtained for both boys and girls was the same
concurs with the fact that in adults, there is a known advantage
to setting up a unisex cut-off point when using the WHtR as a
predictor of cardiovascular disease risk.
9
Hara
et al
.
4
postulated that since WHtR takes into account
children’s height, a single cut-off point can likely be set for the
ratio, without age and gender difference bias.
4
The observed cut-
off value of 0.41 for WHtR from the current study is similar to
what was observed for Japanese children.
4
It is possible that this
is because the children included in the two studies had similar
anthropometric characteristics, and in particular, they were more
or less the same height and had similar WC (Table 2).
On the contrary, results for odds ratio, PPV and NPV show
that the WHtR cut-off value of 0.5 proved to be a better predic-
tor for high blood pressure in children, as shown in Table 5. The
reason for the conflicting results might be due to the fact that
sensitivity and specificity are characteristics of a test and are
not affected by the prevalence of the disease. However, although
the PPV and NPV give a direct assessment of the usefulness of
a test, they are affected by the prevalence of the disease. Bewick
and colleagues
20
stated that ‘the decision to use a diagnostic
test depends not only on the diagnostic accuracy measures but
also on the ultimate benefit to the patient. The prevalence of the
outcome, which is the pre-test probability, must also be known.
Generally, there is a trade-off between sensitivity and specificity,
and the practitioner must make a decision based on their relative
importance’.
20
One potential limitation of our study is that the sample of
children was from only one of nine provinces in South Africa.
Although the children in this study were from both rural, infor-
mal settlements and urban areas, the distribution was not even.
This makes it difficult to suggest the 0.41 cut off for South
African children and adolescents in general. Therefore a nation-
ally representative sample is required to give a more valid cut-off
value.
Another limitation might be the fact that blood pressure
was measured on only one occasion, even though there were
10-minute intervals between the readings. In establishing the
type of hypertension, blood pressure should be measured on
three or more separate occasions.
15
However, the procedure
adopted in this study (vascular unloading technique of Penaz
together with physical criteria of Wesseling) provides reliable,
non-evasive and continuous estimates of blood pressure.
16
Conclusion
Results of the present study indicate that adopting a WHtR cut-
off value lower than 0.5 for South African children may enhance
sensitivity in identifying children at risk for hypertension.
However, a nationally representative cohort study is needed to
confirm or determine a precise cut-off value for accurate predic-
tion of the presence of hypertension. It is important to establish
the diagnostic accuracy of WHtR and hence justify its use in
predicting high blood pressure in the paediatric population.
Identification of children with high blood pressure is important
in controlling the impact of the condition because it allows for
the diagnosis and counselling of persons and facilitates the
implementation of both treatment and management strategies.
This study was supported in part by the South African Sugar Association
grant. Special thanks go to Prof HS Steyn and Dr SM Ellis whose statistical
consultation is gratefully acknowledged.
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