US Childhood Asthma Prevalence Estimates:

The Impact of the 1997 National Health Interview Survey

Redesign

Lara Akinbami, MD

National Center for Health Statistics,

Centers for Disease Control and Prevention

Why track asthma prevalence?

►High impact on pediatric population►Dramatic increase in prevalence over the

past two decades

Childhood asthma: definition and impact

►Chronic disease with episodic airway inflammation and reactivity that causes airflow obstruction

►One of the most common chronic childhood illnesses and a leading cause of disability 14 million missed school days Disproportionate use of health care services

compared to children without asthma2.2 times as many ER visits3.5 times as many hospitalizations

Nearly 200 children die each year

Increasing asthma period prevalence among children 0-17 years of age, 1980-1996

Source: National Health Interview Survey, NCHS, CDC

0

1

2

3

4

5

6

7

8

9

1980 1982 1984 1986 1988 1990 1992 1994 1996

Perc

ent

?

The 1997 redesign of the National Health Interview Survey interrupted monitoring of the asthma period

prevalence trend

Source: National Health Interview Survey, NCHS, CDC

Asthma period prevalence

Asthma attack prevalence

NH

IS r

ede

sig

n

0

1

2

3

4

5

6

7

8

9

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Perc

ent

The National Health Interview Survey (NHIS)

►Continuous household survey of a representative sample of the US civilian noninstitutionalized population

► Face-to-face survey about health conditions and behaviors, including asthma

► In 1997, the NHIS was redesigned to improve the survey

NHIS 1997 redesign

►Elimination of proxy reporting of health conditions (except for children under age 18 years)

►Streamlining data collection on health conditions

The sample is no longer divided between 6 condition lists

Therefore, a larger annual sample of children are asked about asthma

►Change in question wording for chronic conditions: doctor diagnosis required

Evaluating the impact of the 1997 NHIS redesign on childhood asthma

prevalence estimates

► The 1988 NHIS contained the Child Health Supplement (CHS) which included similar questions to those in the 1997 redesigned NHIS

► The 1988 NHIS also included the standard asthma question used before 1997

►Allows comparison of estimates obtained from the “old” standard question to those from questions more similar to the new questions

1980-1996 “old” NHIS

1988 NHIS Child Health Supplement

(CHS)

Redesigned NHIS (1997 onward)

Screener question to determine if child ever had asthma:

(None) Did your child ever have asthma?

Has a doctor or other health professional ever told you that your child had asthma?

Question to estimate asthma prevalence:

During the past 12 months, did anyone in the family have asthma?

IF YES: Did your child have asthma in the past 12 months?

IF YES: During the past 12 months, has your child had an episode of asthma or an asthma attack?

Asthma questions from “old” NHIS, 1988 CHS and redesigned NHIS

Impact of redesign on asthma prevalence estimates on race/ethnicity and age subgroups

1995-96 annual avg period prev

1997 asthma

attack prev

% difference

Overall 6.86 5.44 -20.6

Race

/ethnicityNon Hisp white 6.53 5.22 -20.1

Non Hisp black 8.21 6.75 -17.8

Hispanic 7.61 5.13 -32.5

Age 0-4 yrs 5.03 4.12 -18.1

5-10 yrs 7.43 5.85 -21.3

11-17 yrs 7.74 6.04 -22.0

Study groups 1 and 2 from 1988 NHIS/CHS

►Study Group 1

Responses for standard NHIS asthma question of 1/6 sample (5,509 children)

compared to

Responses for CHS asthma questions of 1/2 sample

(16,624 children)

►Study group 2:

Responses for standard NHIS asthma question compared to those for CHS asthma questions for 1/12 sample with responses to BOTH CHS and standard NHIS (shown in hatched area --2,805 children)

Child health supplement sample

Std

NHIS

sample

Study group 1 Study group 2

Reg CHS % diff Reg CHS % diff

Overall 5.1 4.3 19.4 5.4 4.0 33.8

NH white

0-4 y

5-10 y

11-17 y

3.1

5.7

5.8

2.7

4.9

4.8

14.7

16.7

20.8

2.6

6.8

6.5

2.2

5.0

4.1

19.8

37.5

57.1

NH black

0-4 y

5-10 y

11-17 y

6.0

6.1

7.4

5.2

5.8

4.6

16.1

5.0

60.6

6.2*

7.1*

6.5

5.5

5.1*

5.3*

13.2

37.6

22.1

Hispanic

0-4 y

5-10 y

11-17 y

2.3

5.0

4.9

2.0

4.9

3.6

13.4

1.4

36.6

2.6*

4.8*

3.1*

2.6*

4.8*

3.9*

0

0

-20.9

Differences between standard NHIS and CHS asthma prevalence estimates, 1988

* Relative standard error >30% -- the estimate is unreliable

Calculating inflation factors for the redesigned NHIS estimates

► The differences between 1988 CHS and standard NHIS estimates were used to inflate the redesigned NHIS estimates Method 1 (using study group 1 results) Method 2 (using study group 2 results)

► Two sets of estimates were calculated Overall asthma period prevalence estimates

for 1997-2000 NHIS Race/ethnicity and age group asthma period

prevalence estimates for the 1997-2000 NHIS

1997 1998 1999 2000

Original estimate

5.5 (5.0, 5.9) 5.4 (4.9, 5.9) 5.3 (4.8, 5.8) 5.5 (5.1, 6.0)

Modified estimates

Overall estimate

Method 1

Method 2

6.5 (5.6, 7.5)

7.3 (6.4, 8.2)

6.4 (5.5, 7.4)

7.2 (6.3, 8.1)

6.3 (5.3, 7.3)

7.1 (6.1, 8.0)

6.6 (5.6, 7.6)

7.4 (6.4, 8.3)

Age/race/ethnicity subgroups weighted to estimate alternative overall estimate

Method 1

Method 2

6.5 (4.2, 8.7)

7.2 (4.2, 9.7)

6.5 (4.2, 8.6)

7.1 (4.2, 9.6)

6.3 (4.2, 8.4)

7.0 (4.2, 9.3)

6.7 (4.2, 8.9)

7.3 (4.2, 9.8)

Original asthma attack prevalence and modified estimates and 95% confidence intervals, NHIS,

children 0–17 years

0

1

2

3

4

5

6

7

8

9

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Perc

en

t

Asthma period prevalence (old NHIS)

Asthma attack prevalence (new NHIS)

High modified estimate (method 2)

Low modified estimate (method 1)

Asthma period prevalence (1980-1996) and modified asthma attack prevalence estimates

(1997–2000), NHIS, children 0–17 years

NH

IS r

ede

sig

n

0

1

2

3

4

5

6

7

8

9

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Perc

en

t

Asthma period prevalence (old NHIS)Asthma attack prevalence (new NHIS)High modified estimate (method 2)Low modified estimate (method 1)Current asthma prevalence (new NHIS)

Asthma period prevalence (1980-1996), asthma attack prevalence (1997-2003), modified estimates (1997-

2000) & current asthma prevalence (2001-2003)

NH

IS r

ede

sig

n

Summary--general implications

►Disease prevalence estimates are sensitive to case definition (caveat—there is no way to know the “true” prevalence)

►Surveillance systems using survey instruments may face interrupted trends when surveys undergo periodic revision

►Analysis of trends must include examination of underlying data collection methods

Summary—implications for measuring asthma prevalence

► The 1997 NHIS redesign interrupted the data trend

► The redesigned NHIS measures a different aspect of asthma as reflected in changes in the asthma questions

► “Adjusting” 1997-2000 estimates suggests that asthma prevalence plateaued in the mid 1990s

►New current asthma prevalence estimates are available beginning in 2001, but these also measure a different aspect of asthma and are not comparable to pre-1997 NHIS estimates

Limitations

►Questions from the 1988 CHS and the redesigned NHIS differ

►Changes in question ordering not taken into account

►Survey weights used to estimate asthma prevalence among study group 2 (1/12th sample) were not designed for this purpose

References

1. Newacheck PW, Halfon N. Prevalence, impact and trends in childhood disability due to asthma. Arch Pediatr Adolesc Med 2000;154: 287-293.

2. Weiss KB, Gergen PJ, Hodgson TA. An economic evaluation of asthma in the United States. N Engl J Med 1992;326:862–6.

3. Centers for Disease Control and Prevention. Asthma Prevalence, Health Care Use and Mortality, 2002. http://www.cdc.gov/nchs/products/pubs/pubd/hestats/asthma/asthma.htm

4. Akinbami LJ, Schoendorf KC. Trends in childhood asthma prevalence, health care utilization, and mortality. Pediatrics 2002;110:315–22.

5. Centers for Disease Control and Prevention. Surveillance for Asthma --- United States, 1980—1999. March 29, 2002 / 51(SS01);1-13. http://www.cdc.gov/mmwr/preview/mmwrhtml/ss5101a1.htm

References

6. National Center for Health Statistics. Health measures in the new 1997 redesigned National Health Interview Survey (NHIS). Hyattsville, MD: National Center for Health Statistics, 2000. (http://www.cdc.gov/nchs/about/major/nhis/hisdesgn.htm).

7. Centers for Disease Control and Prevention. Measuring childhood asthma prevalence before and after the 1997 redesign of the National Health Interview Survey—United States. Morb Mort Wkl Rep October 13, 2000 / 49(40);908-911. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm4940a2.htm

8. Akinbami LJ, Schoendorf KC, Parker JD. US Childhood Asthma Prevalence Estimates: The Impact of the 1997 National Health Interview Survey Redesign. Am J Epidemiol 2003;158:99–104.