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دكتر داودخليلي

Cut points of

OBESITY

Dr. KhaliliPhD candidate in epidemiologyShahid beheshti university (MC)

Some Points About Dichotomizing continuous predictors

Simplicity & Practicality Measurement error & low PowerTrade off

Avoid of assumptions

Calculate better effect measurements

We loss some information

Throwing about 1/3 of data away

Some Points About Dichotomizing continuous predictors

Dichotomizing

According to variable distribution

Using a gold standard (usually another variable or event )

Dichotomizing: to create two relatively homogenous group

85% or …of percentile

Cut points based on a Gold Standard

Receiver Operating Characteristic Curve (ROC)

AUC (area Under the Curve)

Sensitivity 1

1 - specificity

Cut points based on a Gold Standard (ROC)

Sensitivity + Specificity -1

Min

Max

Calculating an effect measure (OR, RR, HR, …)

To compare with a reference

Agreement Chart

Cut points based on a Gold Standard

National Health and NutritionExamination Surveys

NHES I 1960-6218-79 years

NHES II 1963-656-11 years

NHES III 1966-7012-17 years

NHANES I 1971-751-74 years

NHANES II 1976-806 mo.-74 years

HHANES 1982-84 6 mo.-74 years

NHANES III 1988-942 mo. +

NHANES 1999-All ages

Survey Dates Ages

OP96025

NIH consensus conference (1985):

·According to NHANES II and 85th percentile values

(men and women ages 20-29 y)

·BMI of 27.8 for men

·BMI of 27.3 for women

05

10152025303540

Men 20-74 y Women 20-74 y

1960-62 1971-74 1976-801988-94 1999-2000

Probloms of this statistical approach:Distribution Changes

Theoretical Curves

Need of more information on BMI complication

Low sensitivity because of underestimation of Obesity

Age-adjusted trends in obesity (BMI >=30): United States

1995 WHO expert committee report

For adults, the Expert Committee proposed classification of BMI with the cut-off points 25, 30 and 40…This classification is based principally on the association between BMI and mortality.

BMI cut-points of 25 (overweight) and 30 (obesity) recommended by expert committees

Relation between mortality and BMI

Data from Lew EA: Mortality and weight: insured lives and the American Cancer Society studies. Ann Intern Med 103:1024-1029, 1985.

The method used to establish BMI cut-off points has been largely arbitrary. In essence, it has been based on visual

inspection of the relationship between BMI and mortality: the cut-off of 30 is based on the point of flexion of the curve.

1998 NHLBI (National Heart, Lung, and Blood Institute )

Clinical Guidelines

In this report, overweight is defined as a BMI of 25.0 to 29.9 kg/m2 and obesity as a BMI of 30 kg/m2. The rationale behind these definitions is based on epidemiological data that show increases in mortality with BMIs above 25 kg/m2. The increase in mortality, however, tends to be modest until a BMI of 30 kg/m2 is reached.

BMI is in itself a strong predictor of overall mortality both above and below the apparent

optimum of about 22·5–25 kg/m2. The progressive excess mortality above this range is

due mainly to vascular disease and is probably largely causal. At 30–35 kg/m2, median

survival is reduced by 2–4 years; at 40–45 kg/m2, it is reduced by 8–10 years (which is

comparable with the effects of smoking). The definite excess mortality below22· 5 kg/m2 is

due mainly to smoking-related diseases, and is not fully explained.

Recent study in western Europe and North America

“Body-mass index and cause-specific mortality in 900 000 adults:collaborative analyses of 57 prospective studies”

Lancet. 2009 March 28; 373(9669): 1083–1096.

Ischaemic heart disease and stroke mortality versus BMI in the range 15–50 kg/m2

Lancet. 2009 March 28; 373(9669): 1083–1096.

Because of some Reasons:WC instead of BMI

WC cut points

According to:

- Lean MEJ, Han TS, Morrison CE. Waist circumference as a measure for indicating need for weight management. BMJ 1995;311:158–61.

Randomly recruited 904 men and 1014 women,aged 25 to 74 years, from the general population ofnorth Glasgow between January and August 1992,excluding only those who were chair bound.

BMI asGold Standard

Using in ATPIII & EGIR

T S Han, EMvan Leer, J C Seidell, ME J LeanWaist circumference action levels in the identification of cardiovascular risk factors: prevalence study in a random sample. BMJ, 1995;311:1401-5

These cutpoints have been shown, in a random sample of 2183 men and 2698 women from the Netherlands, to be associated crosssectionally with an adverse cardiovascular risk profile.

T S Han, EMvan Leer, J C Seidell, ME J Lean1995;311:1401-5

BMI 25 at action level I or 30 at action level 2as Gold standard

T S Han, EMvan Leer, J C Seidell, ME J Lean1995;311:1401-5

ShanKuan Zhu,Am J Clin Nutr 2002;76:743–9.

Current WC cutoffs proposed by the National Institutes ofHealth and the World Health Organization were not chosen on thebasis of their empirical relation to risk factors. Rather, these cutoffswere derived by identifying WC values corresponding to BMIcutoffs for overweight (BMI = 25) or obesity (BMI = 30) (2, 21_).

If WC has an independent or a stronger association with risk factors

than BMI has, then it is inappropriate to base WC thresholdson their association with BMI thresholds. Rather, thresholds foreach should be based on their relation to risk factors. Hence, existingcutoff recommendations may not take full advantage of therelation between WC and obesity-related cardiovascular diseaserisk factors.

WC cutoffs among Chinese adults

Country/ethnic-specific values for WC A Consensus Statement from the IDF

Diabet. Med. 23, 469–480 (2006)

Cohort Studies to determine WC cutoff

Brazil HTN ----- M:87 F:80 2009Australia CVD mortality 20-69 M:96 F:80 2007Japan CVD ≥ 40 M:90 F:80 2009Thailand CHD 35-59 M:82 2007China* CVD risk 18-93 M:83-88 F:76 2007Iran CVD ≥ 40 M:94.5 F:94.5 2009

Country Outcome Age Cut off P.Y.

Different Gold Standard

Different Cut points

The more Hard Outcome with lower prevalence

The higher Cut pointOne prevalent CVD rick factorTwo prevalent CVD rick factorThree prevalent CVD rick factorIncident CVDCVD mortality

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