Lecture 2 - Measures of Morbidity PDF

Measures of morbidity and mortality Dr. Kashif Shafique MBBS (Dow),MPH (Glasgow), PhD (Glasgow) Honorary Clinical Senior Lecturer (University of Glasgow, UK) Associate Professor & Principal School of Public Health...

Measures of morbidity and mortality Dr. Kashif Shafique MBBS (Dow),MPH (Glasgow), PhD (Glasgow) Honorary Clinical Senior Lecturer (University of Glasgow, UK) Associate Professor & Principal School of Public Health Dow University of Health Sciences. Learning objectives: At the end of this lecture you (will) be able to:  List the main measures of morbidity  Define and calculate Morbidity Rates  Recognize the relation between incidence and prevalence rates  Define and calculate Mortality Rates Epidemiologic Research Assumes  Disease occurrence is not random  Systematic investigation of different populations can identify causal and preventive factors  Making comparisons is the cornerstone of systematic investigations  Measures of morbidity  Morbidity rates are used as indicators of health  The extent of illness, injury or disability in a defined population  In epidemiology, the main measures of disease frequency are:  Incidence Rate  Attack Rate  Prevalence What this reflect? 400,00,000 400,000  Incidence rate  Incidence measures the number of new cases of a disease (or other health-related phenomenon) that occur during a specified period of time in a population at risk Incidence rate = numerator denominator The numerator should reflect new cases of a disease which occurred during the specified period. The numerator should not include cases which occurred earlier. The numerator has to come from the population at risk for developing disease (it is a part of the denominator)  The denominator should include persons at risk to develop the disease that is being described during the time period covered.  The denominator does not include persons with the disease.  The denominator may change over time as people develop disease What is the incidence rate from October 1, 1990 to Sep 30, 1991? What is the incidence rate from October 1, 1990 to Sep 30, 1991? 4 4 / 14  Factors affecting incidence rate Real and Artefact  New risk factor  oral contraceptives and increase in thrombo-embolism;  food additives and cancer  New virus (HIV and AIDS)  Changing habits  increased smoking and lung cancer  fluoridated water and decrease in dental caries  Factors affecting incidence rate  Changing virulence of causative organisms - drug-resistant bacteria (TB) - Influenza virus mutation Increase influenza (H1N1) - drug resistance to malaria prophylaxis and increase in malaria  Changing of intervention programmes – vaccination against measles measles – Polio eradication campaigns polio – Chemoprophylaxis meningitis, Rheumatic diseases  Selective migration of susceptible persons to an endemic area incidence  Population pattern - Aging Degenerative diseases  Reporting - Increase reporting incidence  Screening - Early detection of cases incidence  New diagnostic tools - New diagnostic tools detection of cases  Attack Rate An attack rate is a variant of an incidence rate, applied to a narrowly defined population observed for a limited time, such as during an epidemic. The attack rate is usually expressed as % percent. Example Of 76 persons who attended a picnic, 46 subsequently developed gastroenteritis. Calculate the attack rate of gastroenteritis Attendees = 76 gastroenteritis = 46 Attack rate = (46 ÷ 76) X 100 = 61% Prevalence Rate Prevalence measures the number of cases (new and old) of the disease (or other health-related phenomenon) at a point or period in time. point prevalence  Total number of cases(pre - existing and new) at a given point in time x 1000 Total population at the same point in time period prevalence  Total number of cases(pre - existing and new) disease during a given timeperiod x 1000 Total population during the same time period  The numerator for prevalence includes:  all persons ill from a specified cause during a specified time interval (or at a specified point in time)  regardless of when the illness began.  The denominator includes  total population in the same place during a specified time interval (or at a specified point in time) What is the point prevalence on April 1? What is the point prevalence on April 1? 7 / 18  LITTLE EXERCISE Population of a town on March 30, 2013 = 180,000 No. of new active cases of TB occurring between January 1 and June 30, 2013 = 26 No. of active TB cases according to the city register on June 30, 2013 = 264 Calculate the incidence between January 1st to 30th June 2013? Calculate prevalence in this town on 30th June 2013? LITTLE CHALLENGE  A prevalence survey conducted from January 1 through December 31, 2003, identified 1,000 cases of schizophrenia in a city of 2 million persons. The incidence rate of schizophrenia in this population is 5/100,000 persons each year. What percent of the 1,000 cases were newly diagnosed in 2003? Relation between incidence and prevalence Relation between incidence and prevalence Relation between incidence and prevalence Relation between incidence and prevalence  Prevalence ~ incidence x duration of disease − Higher incidence results in higher prevalence − Longer duration results in higher prevalence Several factors may affect prevalence rate  Incidence  Duration of disease  Selective Migration  Disease treatments & outcome  Longer duration Factors of the disease Influencing Prolongation of life of patients Observed Disease without care Shorter Prevalence duration Increase in new cases (increased incidence) of the disease In-migration of cases Higher case-fatality rate of disease Out-migration of noncases Decrease in new cases (decreased incidence) In-migration of susceptible people In-migration of noncases Out-migration of cases Improved diagnostic facilities Out-migration of susceptible people (better reporting) Improved cure rate of cases Divergence between incidence and prevalence Disease in which incidence is stable and prevalence is decreasing  Interpretation Rapid recovery from disease for example,  a new drug has been discovered. The disease is becoming more fatal for example,  an increase in disease virulence,  increasing failure of treatment, or  decreasing application of effective treatment. Selective out migration of cases (perhaps seeking treatment elsewhere). Disease in which incidence is stable And prevalence is increasing Example  A chronic, incurable disease, such as diabetes, can have a low incidence but high prevalence, because the disease is not very fatal—but it cannot be completely cured  Its prevalence is the sum of new and existing cases from past years Interpretation 1. Slow Recovery (the disease is becoming more chronic) due to less effective drugs or Poor compliance (drugs are less frequently used), or resistance to the drugs is increasing. 2. The disease is becoming less fatal due to, Use of a newly discovered, potent drug or the organism is becoming less virulent. Early detection of diseases 3. There is selective immigration of cases to the area. Incidence is increasing over time, but the prevalence is decreasing Example A short-duration, curable disease, such as the common cold, can have a high incidence but low prevalence, because many people get a cold or influenza each year—but it lasts for a short time Interpretation 1. The disease is becoming significantly shorter in duration 2. Better treatment with high cure rate 3. New agent more frequently, more acute. 4. The disease is becoming more fatal.  Carefully and Critically Interpret Percentage of Diabetic Neuropathy 50.00% 45.00% 40.00% 35.00% 30.00% % diabetic 25.00% neuropathy of 20.00% all neuropathies 15.00% 10.00% 5.00% 0.00% 1985 1990 1995 2000 2005 2010 Years Percentage of diabetic neuropathies in Pakistan, 1990– 2009, based on data from the three tertiary care centres. See how it was created  Be-careful with what you see Percentage of births that were extramarital in New Zealand, 1962– 1979, based on data from the Department of Statistics. (Adapted from Benfield J, Kjellstrom T: New Zealand ex-nuptial births and domestic purposes benefits in a different perspective. N Z Nurs J 74:28 Mortality  Mortality risk or rate is an example of incidence where death is the outcome of interest.  The denominator includes both prevalent cases of the disease and individuals who are at risk of developing the disease. Annual mortality rate Epidemiological landscape of diseases AGE-SPECIFIC MORTALITY RATE UNDER FIVE-YEAR ANNUAL MORTALITY CAUSE-SPECIFIC MORTALITY RATE Case-fatality rate Proportional mortality  It is simply the proportion of all deaths that are due to a particular cause for a specified population and time period