Biostatistics and Research Design, #6350 Incidence, Prevalence '10 - '11

Thoughts for the Day: Definition of a Taxpayer: A person who does not have to pass a civil service exam to work for the government! Ronald Reagan “The income tax has made more liars out of the American people than golf has…” Will Rogers '10 - '11

Why are disease rates and risk factors important? Describing your patient community e.g., ethnic mix and disease rates Designing programs e.g., screening program for glaucoma if elderly population e.g., school screenings Evidence for unusual occurrences - epidemics (e.g., cancer, infectious disease (anthrax)) '10 - '11

Incidence: Definitions The probability that healthy people will develop a disease over a specified period of time Rate at which new disease occurs in a group of people who are disease-free Measurement of the rate at which new cases of a disease appear '10 - '11

Incidence numerator = new cases denominator = population at risk specified period of time, usually one year (time component is IMPORTANT!) denominator is usually standardized 100 1,000 10,000 '10 - '11

Where Incidence Comes From: Prospective Cohort Studies Beaver Dam Eye Study: 10 yr incidence of ARMD Orinda Longitudinal Study of Myopia: Tracked ocular components and RE over time (~3 years) Reportable Disease Registries: e.g.: Centers for Disease Control and Prevention; Morbidity and Mortality Weekly Report (MMWR) '10 - '11

Incidence/Risk of Everyday Events Handout: Table 6.1, from: Gerstman, BB. Epidemiology Kept Simple.  2003, Wiley-Liss Inc., Hoboken, New Jersey, p126. Keeps things in perspective! '10 - '11

Incidence: Clinical Example I What is the incidence of contact lens related corneal ulcers? Abstract handout Cheng et al., Incidence of contact-lens-associated microbial keratitis and its related morbidity. Lancet; 1999, 354:181-5. New cases in 3 month period in 1996 Netherlands Nation-wide prevalence of CL wear estimated '10 - '11

MK Examples* Acanthamoeba Pseudomonas aeruginosa * Photos courtesy of Dr. Barry Weissman, UCLA '10 - '11

Incidence: Clinical Example I Results: 92 cases of microbial keratitis Annualized Incidence: 1.1/10,000 for daily wear RGPs 3.5/10,000 for daily wear SCLs 20.0/10,000 for extended wear SCLs Conclusions: Similar to US figures for 1989 (materials not helping) Strongly discourage EW '10 - '11

Incidence: Clinical Example II What is the incidence of contact lens related microbial keratitis in Australia (2008)? Abstract handout Stapleton F, et al., The incidence of contact lens-related microbial keratitis in Australia. Ophthalmol. 2008; 115:1655-62. New cases in 12 month period in 2003-2004 Australia Nation-wide usage of CL wear estimated (for risk factors) '10 - '11

Incidence: Clinical Example II Results: 285 cases of microbial keratitis, 1,798 controls Annualized Incidence: 1.2/10,000 for daily wear RGPs (vs. 1.1 for I) 1.9/10,000 for daily wear SCLs (vs. 3.5 for I) 19.5/10,000 for extended wear SCLs (vs. 20.0 for I) 11.9/10,000 for daily wear Sil-Hydrogel 25.4/10,000 for extended wear Si--Hydrogel Conclusions: Similar to US figures for 1989 (materials not helping) Strongly discourage EW '10 - '11

Prevalence: Definition Probability of people having a particular disease at a given time: cross-sectional studies These are existing cases - influenced by the number of people who have had the disease in the past and the duration of the condition '10 - '11

Prevalence Point prevalence - prevalence at a given point in time NB: sort of like point estimate like mean e.g., MGD: How many during senior rotations (student project)? Period prevalence - prevalence at a point + new cases + recurrences within a given period - most often a year '10 - '11

Prevalence numerator = existing cases denominator = total population at risk point or period denominator is usually standardized often seen per 100,000 '10 - '11

Where Prevalence Comes From: Cross-sectional studies, sometimes cohort studies e.g., Student MGD study e.g., SEE Dry eye study (DE in elderly in MD) Framingham Eye Study (glaucoma, diabetic retinopathy, ARMD etc.) Question generalizability! '10 - '11

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Recent Dry Eye Prevalence Figures Author/ Study Study Method Prevalence Schaumberg et al., 2003 Cross sectional Women only 49-89 yrs USA Questionnaire “Dry” if severe Sx or prior Dx of dry eye n = 36,995 Age-adjusted = 7.8% > age 50 McCarty et al., 1998 Age > 40 yrs Australia Clinical Testing N = 926 7.4% w/two or more signs Schein et al., 1997 Age > 65 n = 2,420 3.5% w/both Sx and low Schirmer OR high RB '10 - '11

Prevalence of MGD by Decade* Overall Prevalence, all ages, = 155 MGD /398 total n  39% n = 61 n = 35 n = 50 n = 43 n = 165 Percent MGD Positive n = 33 n = 11 Age by Decade * Hom et al., Optom Vis Sci; 67:710-712, 1990. '10 - '11

Relationships: the water tank analogy New Cases New Cases Old Cases Incidence (inflow) Prevalence (water level) Former Cases Recovery or death '10 - '11

Comparison of Incidence and Prevalence* Counts new onsets only Level is independent of mean duration Measured as rate or proportion (%) Reflects likelihood of developing disease over time Preferred when studying disease etiology Prevalence Counts “new” and ‘old” cases Depends on mean duration of disease Always a proportion (%) Reflects likelihood of having disease over time Preferred when studying health services utilization * From Gerstman, BB. Epidemiology Kept Simple.  2003, Wiley-Liss, New Jersey, p. 135. '10 - '11

Relationships, Examples Common cold: low prevalence even though incidence is high-Why? Short duration Similarly, bacterial conjunctivitis = low prevalence Chronic Eye Diseases, e.g., glaucoma: High prevalence even though relatively low incidence since disease is chronic '10 - '11

Prevalence: Targeting Rates for Populations Expressed relative to: (all very common): Age Gender Ethnicity '10 - '11

Examples from Literature '10 - '11

The Framingham Eye Study I, Results: Prevalence: Varies by author & criteria!!! Similar to dry eye 1 or both eyes positive*: Cataract  15.5% Diabetic retinopathy  3.1% ARMD  8.8% (P)OAGlaucoma  3.3% * Kahn, et al., Am J Epidemiol, 106:17, 1977. '10 - '11

Examples from Literature Major Eye Studies: from: Hatch, S.W. (ed.) Ophthalmic Research and Epidemiology: Evaluation and Application. © 1998; Butter-Worth Heineman, New York; Tables A-5 and A-6. Demonstrates: Age and prevalence relationship (Table A-6 especially) Variation among studies (differing diagnostic criteria) '10 - '11

Note: Terminology Morbidity = Illness Mortality = Death '10 - '11