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# Measuring Disease Frequency

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Presentation from Dr. Akhilesh Bhargava

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### Measuring Disease Frequency

1. 1. Measuring Disease Frequency Dr. Akhilesh Bhargava MD, DHA, PGDHRM Prof. Community Medicine & Director-SIHFW, Jaipur
2. 2. Measuring Diseases/ Events Epi. Objective: 1. Distribution 2. Comparison with reference to population size and, Time For a. Prioritization b. Trend study c. Resource allocation 2 Akhilesh Bhargava
3. 3. • The fundamental task in epidemiologic research is to quantify the occurrence of illness • The goal is to evaluate causation of illness by relating disease occurrence to characteristics of people and their environment • Rates, Ratios, and Standardized Rates are primary tools for quantifying occurrence of illness 3 Akhilesh Bhargava
4. 4. Counting Tools • Rate • Ratio • Proportion 4 Akhilesh Bhargava
5. 5. Rate? “a measure of speed with which events are occurring in a population in a specified time period.” • A numerator • A denominator that “appropriately” relates the numerator to population at risk • A “unit” such as per 1000, per 100,000 or per million 5 Akhilesh Bhargava
6. 6. Why a rate? • To ensure comparing apples with apples 6 Akhilesh Bhargava
7. 7. Ratio: “a fraction (a/b) of two mutually exclusive groups with elements “a” & “b”. 7 Akhilesh Bhargava
8. 8. Proportion: “a fraction (a /a + b) of two mutually exclusive groups with elements “a”& “b” + 8 Akhilesh Bhargava
9. 9. Index Numerator Denominator Proportion People with All people disease with & without disease Ratio People with People disease without disease Rate People with All people disease in a with & without given period disease 9 Akhilesh Bhargava
10. 10. Counting diseases • Mortality § Tools § Crude Mortality § Case fatality § Proportional Mortality § Standardized Mortality § Age specific Mortality • Morbidity § Tools § Prevalence § Incidence 10 Akhilesh Bhargava
11. 11. Morbidity Measuring: Prevalence- Prevalence - Total no. of cases ( new + old) ----------------- x 100 Total population over specified period Point prevalence Period prevalence Prevalence rate- a Ratio, reflects status 11 Akhilesh Bhargava
12. 12. Population at Risk- in a study of Cancer cervix Total Population All Women 0-25 Yrs Pop. At Risk All Men All 25-69 25-69 Yrs. Women Yrs. 70+ Only Pop. At risk should go into denominator of Prevalence rate but at times it is the total population that is considered 12 Akhilesh Bhargava
13. 13. Prevalence: Prevalence can be expressed either as a proportion or as a rate Expressed as a proportion, prevalence is a number between 0 and 1 As a rate, prevalence can be expressed as per 1000, per 100,000, or per whatever 13 Akhilesh Bhargava
14. 14. Prevalence-Types • Point • Period So far as prevalence is concerned it generally refers to point prevalence. However when the period of observation is large it is referred as period prevalence where the numerator will have all existing cases plus all new cases occurring during period of observation an denominator will be mid year population 14 Akhilesh Bhargava
15. 15. Prevalence: Example In a sample of 1,038 women (70-74 years), 70 were found to have rheumatoid arthritis. The prevalence of arthritis is: 70 P= ------------------ =0.07 per women ( 70-74) 1,038 Or P= 70 per thousand women age 70-74 Or P= 7 percent for women age 70-74 Or………. 15 Akhilesh Bhargava
16. 16. Prevalence: Choice of scale of rate usually depends on the ubiquity of the disease. Thus, more common disease prevalence may be presented as percentage Rare disease prevalence may be presented as per 100,000 or per million 16 Akhilesh Bhargava
17. 17. In 2004 there were 1076 cases of Tuberculosis in District X among 50000 men in age group of 40-44 years. The Prevalence rate will be: 1076 p= ________ =0.0215 per year 50000 = 21.5 per thousand per year = 215 per 10 thousand per year = 2150 per million per year 17 Akhilesh Bhargava
18. 18. Change in Prevalence reflects • Change in incidence or duration of disease • Introduction or impact of an intervention • Selective attrition • Change in disease definition or classification • Significant migration 18 Akhilesh Bhargava
19. 19. Prevalence has its use in – 1. Determination of the sickness load 2. Planning of health services in relation to a. Infrastructure b. Manpower c. Facilities, and d. Finances 3. In making community diagnosis 19 Akhilesh Bhargava
20. 20. Incidence Rate number of new cases occurring during a period of time I= x 100 “total person time” at risk What is “person time”: The duration of time a person is at risk Usually expressed as person years but can be expressed as anything, e.g., person months, person weeks, etc. 20 Akhilesh Bhargava
21. 21. Incidence Rate-Types Cumulative Incidence (CI): New cases --------------------------------------- X 100 Population at risk (PAR) during a specified period Incidence Density (ID)/ Incidence rate New cases ------------------------------------------- X 100 Total Person time of observation in PAR, over a specified period 21 Akhilesh Bhargava
22. 22. “Total Person Time” Sum of person time of all individuals who were at risk and were available for observation. Equivalence of “total person time” 50,000 person years = 5,000 persons observed for 10 years = 1,000 persons observed for 50 years = 10,000 persons observed for 5 years 22 Akhilesh Bhargava
23. 23. Say In 2009 there were 1139 cases of Measles in City “X” (Pop.-2500000, children- 15%) among children 0-5 years. The number of person years was 375000. The incidence rate will be: 1139 I = ------------- =0.00317 per person per year, 375000 or = 3.17466 per thousand per year, or = 31.7066 per 10 thousand per year, or = 3170.666 per million per year To be more accurate, we must add another qualifier, namely, “for children 0-5 years of age” 23 Akhilesh Bhargava
24. 24. This would mean exclusion of a. people currently having disease b. people who had had the disease c. people who are protected on account of-immunization, habits and earlier intervention; from the population at risk 24 Akhilesh Bhargava
25. 25. Incidence Rate: Expressed as- Morbidity rate- New casestotal population at risk Mortality rate- No. Of deaths due to a diseasetotal population Case fatality rate- No. Of deaths due to a diseasetotal no. Of cases of that disease Attack rate- No. Of cases of a disease, not persons / total population at risk for a very short period 25 Akhilesh Bhargava
26. 26. Change in Incidence reflects • Introduction of a new risk factor • Changes in habits • Change in virulence • Change in intervention strategy • Selective migration 26 Akhilesh Bhargava
27. 27. Incidence useful in • surveillance • understanding etiology & pathogenesis & • planning of new services 27 Akhilesh Bhargava
28. 28. Prevalence V/S Incidence Prevalence: A “snapshot” of disease at a point in time in a population Incidence: A description of how new cases of disease are occurring. “force of morbidity” “rate of flow” of cases from non disease to disease state 28 Akhilesh Bhargava
29. 29. Prevalence (P) and Incidence (I) P= I x d d=duration If the disease is stable, that is, if the incidence and duration remains constant over time. 29 Akhilesh Bhargava
30. 30. Pop. On Jan 1, July 1, Dec. 31 2004= 25000 Mid Year Migrated out Died Migrated out Died =100 Cases 30 Akhilesh Bhargava
31. 31. • Point prevalence on Jan.1, 2004 = 400 / 25000 x 100 = 1.6 % • Point prevalence on July, 1, 2004 = 600/ 24800 (i.e. 25000-200) x 100 = 2.41 % • Point prevalence on December 31, 2004 = 400/ 24600 (i.e25000-400) x 100 = 1.62 • Period prevalence in one year = 400+1100 / 25000-200 x 100 = 6.51% • Cumulative Incidence for the year (Jan.1, 2004-December 31, 2003) =1100/ 25000- 400x 100= 4.47 31 Akhilesh Bhargava
32. 32. N= 300 Case no . 1 2 3 R 4 5 6 R July 30, 2003 June30, 2004 R = Date of recurrence Date of Onset of disease Date of Termination or death Point prevalence on July 30, 2003= 4 cases( 1, 2, 3, 6)/ 300 Incidence rate on July 30, 2003= 2 cases (4, 5) /296 Period prevalence between July 30, 2003 to June 30, 2004= 6 /300 32 Akhilesh Bhargava
33. 33. Prevalence Vs. Incidence Prevalence: Relevant for planning of health services Incidence: Relevant for exploring causal theories 33 Akhilesh Bhargava
34. 34. Incidence & Prevalence Rate Type Num erator Denominator Morbidity Incidence New cases Total PAR* Mortality Incidence Deaths due Total population a disease or all causes Case fatality Incidence Deaths due to a No. of case of disease that disease Attack rate Incidence No. of cases of Total PAR for disease lim ited period Period Prevalence No. of Cases Total population prevalence New + Old * population at risk 34 Akhilesh Bhargava
35. 35. Incidence increasing but prevalence decreasing 40 35 30 25 20 Prevalence 15 Incidence 10 5 0 35 Akhilesh Bhargava
36. 36. Interpretation: # Disease duration is reduced and it is becoming acute, or # Disease becoming more fatal 36 Akhilesh Bhargava
37. 37. Incidence stable but prevalence increasing indicates:- 45 40 35 30 25 20 Prevalence 15 Incidence 10 5 0 37 Akhilesh Bhargava
38. 38. Interpretation 1.Slow recovery (disease becoming chronic, drugs less effective) or, 2.Fatality reduced (potent drugs available, new drugs effective) or, 3.Immigration of cases from other area (for better facility available). 38 Akhilesh Bhargava
39. 39. Incidence maintained but prevalence declining means:- 35 30 25 20 15 10 incidence 5 prevalence 0 39 Akhilesh Bhargava
40. 40. # Recovery is becoming rapid, (may be a new drug identified is more effective) # Disease turns into a more fatal one (because of treatment failure, change in virulence, drug resistance) or, # Selective emigration of cases (to seek treatment elsewhere) 40 Akhilesh Bhargava