Measures of Disease Frequency

Incidence

Definition

Incidence refers to the number of new cases of a certain disease or event in a population over a specified period of time. There are 2 ways of measuring incidence: cumulative incidence ( CI CI The percentage of the chest diameter occupied by the heart. Imaging of the Heart and Great Vessels) and incidence rate (IR). Both are measures of the risk of contracting a disease or condition.

1. Cumulative incidence per 1,000 = (Number of new cases of disease occurring in a population over a specified period of time) / (Number of total individuals in the population who are at risk for developing the disease during the specifiedperiod of time) x 1,000:
   
   
   
   • CI is also known as cumulative incidence proportion, incidence proportion, risk, attack rate, or probability of developing the disease.
   • CI is not a true rate but a proportion, as the people included in the numerator are also included in the denominator (total population)
   • The denominator should include only “at-risk“ individuals. Individuals who already have the disease or are incapable of contracting the disease must be excluded from the population.
   • The number used to multiply depends on the frequency of the disease. For example, 1,000 may be sufficient to use for a common disease, but 100,000 or 1,000,000 would be required for a rare disease. Using 100 as the multiplier makes the CI CI The percentage of the chest diameter occupied by the heart. Imaging of the Heart and Great Vessels proportion a percentage.
   • CI CI The percentage of the chest diameter occupied by the heart. Imaging of the Heart and Great Vessels does NOT take into account:
     • People who became lost to follow-up
     • People who die from another cause (a “competing risk”) during the observation period
     • When people developed the disease (i.e., at the beginning, middle, or late in the observation period)
   • The synonym attack rate is commonly used during outbreaks Outbreaks Sudden increase in the incidence of a disease. The concept includes epidemics and pandemics. Influenza Viruses/Influenza of disease and in hypothetical predictions.
     • Food-specific attack rate per 100 = (Number of susceptible or at-risk individuals who were exposed to a pathogen or suspect food who became ill) / (Number of total individuals who were exposed to that pathogen or food)
     • Example: If 30 people developed gastroenteritis Gastroenteritis Gastroenteritis is inflammation of the stomach and intestines, commonly caused by infections from bacteria, viruses, or parasites. Transmission may be foodborne, fecal-oral, or through animal contact. Common clinical features include abdominal pain, diarrhea, vomiting, fever, and dehydration. Gastroenteritis after eating potato salad, and 100 people in total ate the salad, then the AR AR Aortic regurgitation (AR) is a cardiac condition characterized by the backflow of blood from the aorta to the left ventricle during diastole. Aortic regurgitation is associated with an abnormal aortic valve and/or aortic root stemming from multiple causes, commonly rheumatic heart disease as well as congenital and degenerative valvular disorders. Aortic Regurgitation = 30 / 100 x 100 = 30%, which is the risk of becoming ill after eating the salad.
   • CI CI The percentage of the chest diameter occupied by the heart. Imaging of the Heart and Great Vessels is used mostly used in situations where the follow-up time is relatively short and there is relatively little loss to follow-up. Otherwise, epidemiologists generally use the incidence rate.
     
2. Incidence rate per 1,000 = (Number of new cases of disease occurring in the population during a specified period of time) / (Total person-time, or the sum of the time periods of observation of each person who was observed) x 1,000
   
   
   
   • IR is a true rate and is also known as person-time rate or incidence density.
   • Total person-time: based on the eligibility criteria for the study population that an individual must be at risk (disease-free) and actively followed. Once a person develops the disease or is lost to follow-up, their contribution of time at risk ends. If their contribution ends after 1 year, then that person has contributed 1 person-year (py) to the study. For example:
     • 1 individual at risk who is in a 1-year study = 1 py
     • 5 individuals at risk who are in a 5-year study = 25 py
     • 5 individuals at risk who are in a 5-year study, with 2 being lost to follow-up or disease after 1 year while the remaining 3 continue to be at risk for the full 5 years = 2 py + (3 x 5) = 17 py

Examples

Cumulative incidence

Over the past year at a particular hospital, 500,000 patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship were admitted for hip or knee replacements. Five thousand of those patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship over that year had a venous thromboembolism Thromboembolism Obstruction of a blood vessel (embolism) by a blood clot (thrombus) in the blood stream. Systemic Lupus Erythematosus ( VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States) during their admission. What is the risk of developing VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States over the past year at this hospital? 

• There are 5,000 new cases of VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States over the year in a population at risk of 500,000.
  
  
  
• Therefore, 1% of those admitted, or 1 out of 100 patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship, experienced VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States during their hospitalization Hospitalization The confinement of a patient in a hospital. Delirium.

Incidence rate

Over the time period from 2000 to 2009, in Quebec, Canada, there were 91,000 cases of VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States over a population, with a disease-free time over that period that totals 74,200,000 py. What is the IR of VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States in Quebec over this time period?

Therefore, the IR is 0.0012 cases per py. By convention, this is multiplied by 10,000 to get 12 cases per 10,000 py of observation.

Prevalence

Definition

• Prevalence is the number of affected individuals present in a population at a particular point in time divided by the number of total individuals in the population at that point in time (point prevalence) or over a period of time (period prevalence). Prevalence is typically expressed as a proportion or percentage.
  
  
  
• Point prevalence is like a “snapshot” of the population that determines who has the disease and who does not; much more commonly used than period prevalence.
• Does not measure risk, but provides an important measure of the burden of disease in a community, which is necessary for resource allocation.
  • Prevalence cannot provide a risk estimation, as it does not give information about when the disease developed. Some individuals may have developed the disease a week ago, some last year, and some 20 years ago.
  • There is a relationship between prevalence and incidence (Prevalence = Incidence × Duration of disease) as long as:
    • The population is in a steady state Steady state Enzyme Kinetics.
    • The disease rate is constant.
    • The prevalence is not too high.

Example

Prevalence versus incidence

• In 2 hypothetical cities, with 1,000 inhabitants in each, there is a contagious disease with a prevalence of 100 / 1,000 in City A and a prevalence of 60 / 1,000 in City B.
• There is not enough information to determine which city would be “less risky” to live in without considering incidence.
• However, if City B has an incidence of 20 cases / year, while City A has only 4 cases / year, then City B is riskier.
• The prevalence in City B is lower because the average survival of a diseased person in City B is only 3 years, while a diseased person in City A lives an average of 25 years.
  • For City A:  4 / year x 25 years = 100 / 1,000
  • For City B: 20 / year x 3 years = 60 / 1,000

Period prevalence

Over the past year, there were 12 million patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship in the Medicare database, and 200,000 Medicare patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship suffered a VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States. What is the period prevalence of VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States in Medicare patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship in the United States over the past year?

• The period prevalence for the year is a measure of the proportion of Medicare patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship who had VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States. There were 12 million patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship in the Medicare database; 200,000 of these patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship had VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States.
  
  
  
• Therefore, the period prevalence of VTE VTE Obstruction of a vein or veins (embolism) by a blood clot (thrombus) in the bloodstream. Hypercoagulable States in Medicare patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship for the year was 1.6%.