Section 2: Morbidity Frequency MeasuresMorbidity has been defined as any departure, subjective or objective, from a state of physiological or psychological well-being. In practice, morbidity encompasses disease, injury, and disability. In addition, although for this lesson the term refers to the number of persons who are ill, it can also be used to describe the periods of illness that these persons experienced, or the duration of these illnesses.(4) Show
Measures of morbidity frequency characterize the number of persons in a population who become ill (incidence) or are ill at a given time (prevalence). Commonly used measures are listed in Table 3.3. Table 3.3 Frequently Used Measures of Morbidity
Incidence refers to the occurrence of new cases of disease or injury in a population over a specified period of time. Although some epidemiologists use incidence to mean the number of new cases in a community, others use incidence to mean the number of new cases per unit of population. Two types of incidence are commonly used — incidence proportion and incidence rate. Incidence proportion or riskDefinition of incidence proportionSynonyms for incidence proportion
Incidence proportion is the proportion of an initially disease-free population that develops disease, becomes injured, or dies during a specified (usually limited) period of time. Synonyms include attack rate, risk, probability of getting disease, and cumulative incidence. Incidence proportion is a proportion because the persons in the numerator, those who develop disease, are all included in the denominator (the entire population). Method for calculating incidence proportion (risk)Number of new cases of disease or injury during EXAMPLES: Calculating Incidence Proportion (Risk)Example A: In the study of diabetics, 100 of the 189 diabetic men died during the 13-year follow-up period. Calculate the risk of death for these men. Numerator = 100 deaths among the diabetic men Risk = (100 ⁄ 189) × 100 = 52.9% Example B: In an outbreak of gastroenteritis among attendees of a corporate picnic, 99 persons ate potato salad, 30 of whom developed gastroenteritis. Calculate the risk of illness among persons who ate potato salad. Numerator = 30 persons who ate potato salad and developed gastroenteritis Risk = “Food-specific attack rate” = (30 ⁄ 99) × 100 = 0.303 × 100 = 30.3% Properties and uses of incidence proportions
More About DenominatorsThe denominator of an incidence proportion is the number of persons at the start of the observation period. The denominator should be limited to the “population at risk” for developing disease, i.e., persons who have the potential to get the disease and be included in the numerator. For example, if the numerator represents new cases of cancer of the ovaries, the denominator should be restricted to women, because men do not have ovaries. This is easily accomplished because census data by sex are readily available. In fact, ideally the denominator should be restricted to women with ovaries, excluding women who have had their ovaries removed surgically (often done in conjunction with a hysterectomy), but this is not usually practical. This is an example of field epidemiologists doing the best they can with the data they have.
Often, the total number of contacts in the denominator is calculated as the total population in the households of the primary cases, minus the number of primary cases. For a secondary attack rate, 10n usually is 100%. EXAMPLE: Calculating Secondary Attack RatesConsider an outbreak of shigellosis in which 18 persons in 18 different households all became ill. If the population of the community was 1,000, then the overall attack rate was 18 ⁄ 1,000 × 100% = 1.8%. One incubation period later, 17 persons in the same households as these “primary” cases developed shigellosis. If the 18 households included 86 persons, calculate the secondary attack rate. Secondary attack rate = (17 ⁄ (86 − 18)) × 100% = (17 ⁄ 68) × 100% = 25.0% Incidence rate or person-time rateDefinition of incidence rateIncidence rate or person-time rate is a measure of incidence that incorporates time directly into the denominator. A person-time rate is generally calculated from a long-term cohort follow-up study, wherein enrollees are followed over time and the occurrence of new cases of disease is documented. Typically, each person is observed from an established starting time until one of four “end points” is reached: onset of disease, death, migration out of the study (“lost to follow-up”), or the end of the study. Similar to the incidence proportion, the numerator of the incidence rate is the number of new cases identified during the period of observation. However, the denominator differs. The denominator is the sum of the time each person was observed, totaled for all persons. This denominator represents the total time the population was at risk of and being watched for disease. Thus, the incidence rate is the ratio of the number of cases to the total time the population is at risk of disease. Method for calculating incidence rateNumber of new cases of disease or injury during specified period Time each person was observed, totaled for all personsIn a long-term follow-up study of morbidity, each study participant may be followed or observed for several years. One person followed for 5 years without developing disease is said to contribute 5 person-years of follow-up. What about a person followed for one year before being lost to follow-up at year 2? Many researchers assume that persons lost to follow-up were, on average, disease-free for half the year, and thus contribute ½ year to the denominator. Therefore, the person followed for one year before being lost to follow-up contributes 1.5 person-years. The same assumption is made for participants diagnosed with the disease at the year 2 examination — some may have developed illness in month 1, and others in months 2 through 12. So, on average, they developed illness halfway through the year. As a result, persons diagnosed with the disease contribute ½ year of follow-up during the year of diagnosis. The denominator of the person-time rate is the sum of all of the person-years for each study participant. So, someone lost to follow-up in year 3, and someone diagnosed with the disease in year 3, each contributes 2.5 years of disease-free follow-up to the denominator. Properties and uses of incidence rates
EXAMPLES: Calculating Incidence RatesExample A: Investigators enrolled 2,100 women in a study and followed them annually for four years to determine the incidence rate of heart disease. After one year, none had a new diagnosis of heart disease, but 100 had been lost to follow-up. After two years, one had a new diagnosis of heart disease, and another 99 had been lost to follow-up. After three years, another seven had new diagnoses of heart disease, and 793 had been lost to follow-up. After four years, another 8 had new diagnoses with heart disease, and 392 more had been lost to follow-up. The study results could also be described as follows: No heart disease was diagnosed at the first year. Heart disease was diagnosed in one woman at the second year, in seven women at the third year, and in eight women at the fourth year of follow-up. One hundred women were lost to follow-up by the first year, another 99 were lost to follow-up after two years, another 793 were lost to follow-up after three years, and another 392 women were lost to follow-up after 4 years, leaving 700 women who were followed for four years and remained disease free. Calculate the incidence rate of heart disease among this cohort. Assume that persons with new diagnoses of heart disease and those lost to follow-up were disease-free for half the year, and thus contribute ½ year to the denominator. Numerator = number of new cases of heart disease Denominator = person-years of observation or Denominator = person-years of
observation Person-time rate = Number of new cases of disease or injury during specified period Time each person was observed, totaled for all persons= 16 ⁄ 6,400 In contrast, the incidence proportion can be calculated as 16 ⁄ 2,100 = 7.6 cases per 1,000 population during the four-year period, or an average of 1.9 cases per 1,000 per year (7.6 divided by 4 years). The incidence proportion underestimates the true rate because it ignores persons lost to follow-up, and assumes that they remained disease-free for all four years. Example B: The diabetes follow-up study included 218 diabetic women and 3,823 nondiabetic women. By the end of the study, 72 of the diabetic women and 511 of the nondiabetic women had died. The diabetic women were observed for a total of 1,862 person years; the nondiabetic women were observed for a total of 36,653 person years. Calculate the incidence rates of death for the diabetic and non-diabetic women. For diabetic women, numerator = 72 and denominator = 1,862 Person-time rate = 72 ⁄ 1,862 For nondiabetic women, numerator = 511 and denominator = 36,653 Person-time rate = 511 ⁄ 36,653 = 0.0139 deaths per person-year EXAMPLES: Calculating Incidence Rates (Continued)Example C: In 2003, 44,232 new cases of acquired immunodeficiency syndrome (AIDS) were reported in the United States.(5) The estimated mid-year population of the U.S. in 2003 was approximately 290,809,777.(6) Calculate the incidence rate of AIDS in 2003. Numerator = 44,232 new cases of AIDS Incidence rate = (44,232 ⁄ 290,809,777) × 100,000 PrevalenceDefinition of prevalencePrevalence, sometimes referred to as prevalence rate, is the proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time. Prevalence differs from incidence in that prevalence includes all cases, both new and preexisting, in the population at the specified time, whereas incidence is limited to new cases only. Point prevalence refers to the prevalence measured at a particular point in time. It is the proportion of persons with a particular disease or attribute on a particular date. Period prevalence refers to prevalence measured over an interval of time. It is the proportion of persons with a particular disease or attribute at any time during the interval. Method for calculating prevalence of diseaseAll new and pre-existing cases × 10 n Method for calculating prevalence of an attributePersons having a particular attribute × 10 n The value of 10 n is usually 1 or 100 for common attributes. The value of 10 n might be 1,000, 100,000, or even 1,000,000 for rare attributes and for most diseases. EXAMPLE: Calculating PrevalenceIn a survey of 1,150 women who gave birth in Maine in 2000, a total of 468 reported taking a multivitamin at least 4 times a week during the month before becoming pregnant.(7) Calculate the prevalence of frequent multivitamin use in this group. Numerator = 468 multivitamin users Prevalence = (468 ⁄ 1,150) × 100 = 0.407 × 100 = 40.7% Properties and uses of prevalence
EXAMPLES: Incidence versus PrevalenceFigure 3.1 represents 10 new cases of illness over about 15 months in a population of 20 persons. Each horizontal line represents one person. The down arrow indicates the date of onset of illness. The solid line represents the duration of illness. The up arrow and the cross represent the date of recovery and date of death, respectively. Figure 3.1 New Cases of Illness from October 1, 2004–September 30, 2005 Image Description Example A: Calculate the incidence rate from October 1, 2004, to September 30, 2005, using the midpoint population (population alive on April 1, 2005) as the denominator. Express the rate per 100 population. Incidence rate numerator = number of new cases between October 1 and September 30 Incidence rate denominator = April 1 population Incidence rate
= (4 ⁄ 18) × 100 Example B: Calculate the point prevalence on April 1, 2005. Point prevalence is the number of persons ill on the date divided by the population on that date. On April 1, seven persons (persons 1, 4, 5, 7, 9, and 10) were ill. Point prevalence = (7 ⁄ 18) × 100 Example C: Calculate the period prevalence from October 1, 2004, to September 30, 2005. The numerator of period prevalence includes anyone who was ill any time during the period. In Figure 3.1, the first 10 persons were all ill at some time during the period. Period prevalence = (10 ⁄ 20) × 100 Exercise 3.2For each of the fractions shown below, indicate whether it is an incidence proportion, incidence rate, prevalence, or none of the three.
Check your answer. References (This Section)
Figure 3.1Description: Before October 1, six people became ill; 2 of them died before April 1. Between October 1 and September 30, four more persons became ill. Six more persons died after April 1. Return to text. Which term refers to techniques and tests used to identify a disease?Listen to pronunciation. (DY-ug-NOH-sis) The process of identifying a disease, condition, or injury from its signs and symptoms. A health history, physical exam, and tests, such as blood tests, imaging tests, and biopsies, may be used to help make a diagnosis.
What is most helpful to the doctor in determining a patient's likely diagnosis?Medical history. The medical history of a patient is the most useful and important element in making an accurate diagnosis, much more valuable than either physical examinations or diagnostic tests.
Which of the following statements is the best definition for differential diagnosis?A differential diagnosis is a list of possible conditions that share the same symptoms that you described to your healthcare provider. This list is not your final diagnosis, but a theory as to what is potentially causing your symptoms.
Can codes for symptoms signs and ill defined conditions be sequenced as a principal diagnosis?Codes for symptoms, signs, and ill-defined conditions from Chapter 18 of the ICD-10-CM coding manual cannot be used as principal diagnosis or reasons for outpatient encounters when related diagnosis has been established, they may be assigned as an additional diagnosis.
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