Epidemiology
Sam Asgarian, M4 Tulane University
BE01- 1 !
Epidemiology advice •
Easy section = easy to overlook
•
Points here can add up: make sure to prep as you do for any Step 1 section and do not cram ! get all these points on Test Day!
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Become more ingrained in the test
FA 2012: n/a
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FA 2011: n/a
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FA 2010: n/a
BE01- 2
Incidence Incidence rate: the rate at which new events occur in a population.The numerator is the number of NEW events that occur in a defined period; the denominator is the population at risk of experiencing this new event during the same period.
Incidence =
# of new events in events in a specific period of time # “at risk” of having a new event in the specific time period
a. Attack rate: a rate: a type of incidence rate in which the denominator is further reduced for some known exposure
b. Focus on acute acute conditions conditions
FA 2012: 54.1
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FA 2011: 52.1 ME 3e: 34
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FA 2010: 54
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BE01- 3
Prevalence Prevalence rate: all persons who experience an event in a population. The numerator is ALL individuals who have an attribute or disease at a particular point in time (or during a particular period of time); the denominator is the population at risk of having the attribute or disease at this point in time or midway through the period.
# of individuals with disease/attribute
Prevalence =
Total population at risk
• Point prevalence: prevalence at a specified point in time • Period prevalence: prevalence during a specified period or span of time • Focus on chronic conditions
FA 2012: 54.1
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FA 2011: 52.1 ME 3e: 34
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FA 2010: 54
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BE01- 4
Clinical Trials •
Clinical Trials 1. Control group: subjects who do not receive the intervention under study; used as a source of comparison to be certain that the experiment group is being affected by the intervention and not by other factors. In clinical trials, this is most often a placebo group. 2. For Food and Drug Administration (FDA) approval, three phases of clinical trials must be passed. a. Phase One: testing safety in healthy volunteers b. Phase Two: testing protocol and dose levels in a small group of patient volunteers c. Phase Three: testing efficacy and occurrence of side effects in a larger group of patient volunteers. Phase III is considered the definitive test. d. Post-marketing Survey: collecting reports of drug side effects when out in common usage (post-FDA approval)
FA 2012: 52.2
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FA 2011: 50.2
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FA 2010: 52
BE01- 5
Case-control and Cross-sectional studies • •
STUDIES: Case-control study: identifies a group of people with the disease and compares them with a suitable comparison group without the disease. Almost always retrospective (e.g., comparing cases of treatment-resistant TB with cases of nonresistant TB) a. Cannot assess incidence or prevalence of disease b. Can help determine causal relationships c. Very useful for studying conditions with very low incidence or prevalence
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Cross-sectional study: the presence or absence of disease and other variables are determined in each member of the study population or in a representative sample at a particular time. The co-occurrence of a variable and the disease can be examined. a. Disease prevalence rather than incidence is recorded b. The temporal sequence of cause and effect cannot usually be determined in a cross-sectional study c. Example: who in the community now has treatment-resistant TB
FA 2012: 52.1
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FA 2011: 50.1 ME 3e: 35
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FA 2010: 52
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BE01- 6
Cohort Study Cohort study: population group of those who have been exposed to risk factor is identified and followed over time and compared with a group not exposed to the risk factor. Outcome is disease incidence in each group (e.g., following a group of individuals forward in time and comparing the development of lung cancer in those who smoke and in those who don’t smoke).
• Prospective; subjects tracked forward in time
• Can determine incidence and causal relationships
• Must follow population long enough for incidence to appear
FA 2012: 52.1
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FA 2011: 50.1 ME 3e: 34
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FA 2010: 52
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BE01- 7
Difference between studies by time
Differentiating Study Types by Time
FA 2012: 52.1
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FA 2011: 50.1 ME 3e: 35
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FA 2010: 52
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BE01- 8
Twin and Adoption Studies •
Measuring inheritance of a genetic disease/disorder: • Twins: monozygotic and dizygotic • Adoption: nature vs. nurture
FA 2012: 52.1
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FA 2011: 50.1
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FA 2010: 52
BE01- 9
Diagnostic Tests Diagnostic Tests – Will be covered on the exam –
Pregnancy test example (don’t want false positive, but really don’t want a false negative ! will take risks, etc)
– Know this table (Table 1-4):
FA 2012: 53.1
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FA 2011: 51.1
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FA 2010: 53
BE01- 10
Sensitivity Sensitivity: the probability of correctly identifying a case of disease. Sensitivity is the proportion of truly diseased persons in the screened population who are identified as diseased by the screening test. This is also known as the “true positive rate.”
TP
Sensitivity =
TP+FN
true positives/(true positives + false negatives)
– Measures only the distribution of persons with disease – Uses data from the left column (Table 1-4) – Note: 1-sensitivity = false negative rate
FA 2012: 53.1
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FA 2011: 51.1 ME 3e: 34
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FA 2010: 53
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BE01- 11
Specificity Specificity: the probability of correctly identifying disease-free persons. Specificity is the proportion of truly nondiseased persons who are identified as nondiseased by the screening test. This is also known as the “true negative rate.”
TN
Specificity =
TN+FP
true negatives/(true negatives + false positives)
– Measures only the distribution of persons who are disease-free – Uses data from the right column (Table 1-4) – Note: 1-specificity = false positive rate
FA 2012: 53.1
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FA 2011: 51.1 ME 3e: 34
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FA 2010: 53
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BE01- 12
Positive Predictive Value Positive predictive value: the probability of disease in a person who receives a positive test result. The probability that a person with a positive test is a true positive. (i.e., has the disease) is referred to as the “predictive value of a positive test.”
Positive predictive value =
TP TP+FP
true positives/(true positives + false positives)
– Measures only the distribution of persons who receive a positive test result – Uses data from the top row of the (Table 1-4)
FA 2012: 53.1
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FA 2011: 51.1 ME 3e: 34
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FA 2010: 53
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BE01- 13
Negative Predictive Value Negative predictive value: the probability of no disease in a person who receives a negative test result. The probability that a person with a negative test is a true negative (i.e., does not have the disease) is referred to as the “predictive value of a negative test.”
Negative predictive value =
TN TN+FN
true negatives/(true negatives + false negatives)
– Measures only the distribution of persons who receive a negative test result – Uses data from the bottom row (Table 1-4)
FA 2012: 53.1
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FA 2011: 51.1 ME 3e: 34
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FA 2010: 53
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BE01- 14
Meta-analysis •
Meta-analysis: a. A statistical way of combining the results of many studies to produce one overall conclusion b. A mathematic literature review – Highest level of clinical evidence used to make an argument for or against something
FA 2012: 58.2
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FA 2011: 50.3
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FA 2010: 52
BE01- 15
Accuracy and Precision Accuracy: total percentage correctly selected; the degree to which a measurement, or an estimate based on measurements, represents the true value of the attribute that is being measured.
Accuracy =
TP + TN TP + FP + TN + FN
(true positives + true negatives)/total screened patients
Precision: ability of a test to measure something consistently, either across testing situations (test-retest reliability), within a test (split-half reliability), or across judges (inter-rater reliability) – Reliability: think of the clustering of rifle shots at a target
FA 2012: 55.1
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FA 2011: 52.3 ME 3e: 34
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FA 2010: 54
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BE01- 16
Odds Ratio Odds ratio: looks at the increased odds of getting a disease with exposure to a risk factor versus nonexposure to that factor Disease Present (+)
Disease Absent (-)
Exposed group (risk factor +)
A
B
Nonexposed group (risk factor -, control)
C
D
Odds Ratio = A/B C/D
=
AD BC
– Odds of having disease in exposed group divided by odds of having disease in nonexposed group – The odds that a person with lung cancer was a smoker versus the odds that a person without lung cancer was a smoker – Odds ratio does not so much predict disease as estimate the strength of a risk factor FA 2012: 54.2
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FA 2011: 52.2 ME 3e: 35
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FA 2010: 54
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BE01- 17
Relative Risk Relative risk (RR): comparative probability asking “How much more likely?” Disease Present (+)
Disease Absent (-)
Exposed group (risk factor +)
A
B
Nonexposed group (risk factor -, control)
C
D
Relative risk (RR) = A/(A+B) C/(C+D) – Incidence rate of exposed group divided by the incidence rate of the unexposed group – How much greater chance does one group have of contracting the disease compared with the other group? – For statistical analysis, yields a p-value
FA 2012: 54.2
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FA 2011: 52.2 ME 3e: 35
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FA 2010: 54
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BE01- 18
Attributable Risk Attributable risk (AR): comparative probability asking “How many more cases in one group?” Disease Present (+)
Disease Absent (-)
Exposed group (risk factor +)
A
B
Nonexposed group (risk factor -, control)
C
D
Attributable risk (AR) =
A C ! A+B C+D
– Incidence rate of exposed group minus the incidence rate of the unexposed group – Note that both relative risk and attributable risk tell us if there are differences, but do not tell us why those differences exist.
FA 2012: 54.2
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FA 2011: 52.2 ME 3e: 35
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FA 2010: 54
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BE01- 19
Types of Bias in Research
FA 2012: 56.1
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FA 2011: 53.1 ME 3e: 35
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FA 2010: 55
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BE01- 20
Bias: Selection, Measurement, Experimenter Expectancy Selection: sample selected is NOT representative of the population • Heart disease study with participants from health club • AKA: Berkson’s bias prevalence
! used
hospital records to estimate population
Measurement: info gathered is distorted • “You don’t like your doctor, do you?” as a leading question for patient satisfaction • Hawthorne effect: being studied changes the behavior (similar to being recorded)
Experimenter expectancy: experimenter communicates (verbal or non-verbal) to the subjects who then produce the desired effects (Pygmalion Effect) FA 2012: 56.1
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FA 2011: 53.1 ME 3e: 35
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FA 2010: 55
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BE01- 21
Bias: Lead-time, Recall, Late-look, Confounding •
Lead-time bias: – False estimate of survival rates • Seem to live longer with a disease after a positive screening test • Dz discovered sooner, so patients SEEM to live longer
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Recall bias: – Fail to accurately recall events in the past (retrospective studies)
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Late-look bias: – Survey doesn’t uncover patients with SEVERE disease; uses mild and moderate cases to come to a conclusion; persons with AIDS report mild symptoms (the severe ones are not alive to be counted in the survey)
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Confounding bias: – Factor examined is related to other factors; heart disease in exercise; if one group is young, the other is old ! heart dz could be due to age, not exercise
FA 2012: 56.1
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FA 2011: 53.1 ME 3e: 35
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FA 2010: 55
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BE01- 22
Design Bias •
Design bias: – Parts of the study do not fit together; non-comparable control group; measuring anti-hypertensive drug effect between those with high blood pressure and those with normal blood pressure
FA 2012: 56.1
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FA 2011: 53.1 ME 3e: 35
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FA 2010: 55
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BE01- 23
Biostatistics
Sam Asgarian, M4 Tulane University
BE02- 1 !
Hypothesis Testing •
Hypothesis Testing: a. Define the research question: what are you trying to show? b. Define the null hypothesis, generally the opposite of what you hope to show i. Null hypothesis says that the findings are the result of chance or random factors. If you want to show that a drug works, the null hypothesis will be that the drug does NOT work. – We never accept the null hypothesis. We either reject it or fail to reject it. Saying we do not have sufficient evidence to reject it is not the same as being able to affirm that it is true. ii. Alternative hypothesis says what is left after defining the null hypothesis. In this example, that the drug does actually work.
FA 2012: 57.2
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FA 2011: 54.1 ME 3e: 35
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FA 2010: 56
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BE02- 2
Type I and Type II Errors •
Errors: i.
Type I error (alpha error): rejecting the null hypothesis when it is really true (i.e., assuming a statistically significant effect on the basis of the sample when there is none in the population, e.g., asserting that the drug works when it doesn’t). The chance of type I error is given by the p-value. If p = 0.05, then the chance of a type I error is 5 in 100, or 1 in 20.
ii.
Type II error (beta error): failing to reject the null hypothesis when it is really false (i.e., declaring no significant effect on the basis of the sample when there really is one in the population, e.g., asserting the drug does not work when it really does). The chance of a type II error cannot be directly estimated from the p-value.
FA 2012: 57.3
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FA 2011: 54.2 ME 3e: 35
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FA 2010: 56
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BE02- 3
Power •
Power is directly related to type II error: 1 – ! = Power – Just as increasing the power of a microscope makes it easier to see what is going on in histology, increasing statistical power allows us to detect what is happening in the data. – There are a number of ways to increase statistical power. The most common is to increase the sample size.
FA 2012: 58.1
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FA 2011: 54.3 ME 3e: 35
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FA 2010: 56
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BE02- 4
Skewed vs. Normal curves •
Skewed curves: not all curves are normal
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Normal is when mean = median = mode (bell curve) – Sometimes the curve is skewed either positively or negatively – A positive skew has the tail to the right and the mean greater than the median – A negative skew has the tail to the left and the median greater than the mean – For skewed distributions, the median is a better representation of central tendency than is the mean
FA 2012: 57.1
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FA 2011: 53.2
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FA 2010: 55
BE02- 5
T-test •
T-test – Comparing the means of two groups from a single nominal variable, using means from an interval variable to see whether the groups are different – Used for two groups only (i.e., compares two means, e.g., do patients with MI who are in psychotherapy have a reduced length of convalescence compared with those who are not in therapy?)
FA 2012: 58.4
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FA 2011: 55.2 ME 3e: 36
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FA 2010: 57
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BE02- 6
ANOVA •
ANOVA (Analysis of variance) – One-way: compares means of many groups (two or more) of a single nominal variable using an interval variable; significant p-value means that at least two of the tested groups are different
FA 2012: 58.4
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FA 2011: 55.2 ME 3e: 36
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FA 2010: 57
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BE02- 7
Chi-Squared •
Chi-Squared – Tests to see whether two nominal variables (NOT MEAN) are independent (e.g., testing the efficacy of a new drug by comparing the number of recovered patients given the drug with those who are not)
FA 2012: 58.4
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FA 2011: 55.2 ME 3e: 36
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FA 2010: 57
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BE02- 8
Disease Prevention •
Disease Prevention – 3 stages of prevention 1) Prevent occurrence – Condoms and HIV 2) Early detection – Saliva swab, blood test, etc. 3) Reduce disease effects – Anti-retroviral therapy (prevent HIV ! AIDS)
FA 2012: 58.6
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FA 2011: 55.4
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FA 2010: 57
BE02- 9
Reportable Diseases & Leading Causes of Death •
Speaking of AIDS " – Reportable diseases: • AIDS is reportable everywhere, HIV varies • Hep A, B and C; measles, mumps, rubella (MMR) • Salmonella, TB, chickenpox – Leading causes of death • Infant = congenital (NOT same as genetic), respiratory • Age 1-14 = injuries, cancer, homicide • Age 15-24 = injuries, homicide, suicide • Ages 25-64 = cancer, heart dz, injuries • Ages 65 and older = heart disease, cancer, stroke
FA 2012: n/a
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FA 2011: 56.1
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FA 2010: 58
BE02- 10
Suicide •
Leading causes of death " suicide – Between 10 and 20 suicide attempts for every one that succeeds • Men commit suicide four times as often as women • Women attempt suicide three times as often as men – Firearms are the most likely method by which either men or women commit suicide. Pills/poisons most likely method for women to attempt suicide – Suicides outnumber homicides in the U.S.
FA 2012: n/a
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FA 2011: 56.2
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FA 2010: 58
BE02- 11
Medicare and Medicaid •
Medicare and Medicaid – Care for elderly, no one aids the young " • Resident salaries/stipends come from Medicare/Medicaid
FA 2012: 59.1
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FA 2011: 56.3
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FA 2010: 58
BE02- 12
Ethics
Sam Asgarian, M4 Tulane University
BE03- 1
Physicians’ Ethics •
Physicians’ Ethics: 1) Beneficence = Do good 2) Nonmaleficence = Do no harm 3) Justice = Equality 4) Autonomy (wins out) = Not paternalistic
FA 2012: 59.2
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FA 2011: 57.1 ME 3e: 30
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FA 2010: 59
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BE03- 2
Informed Consent •
Informed Consent – Risks/benefits/alternatives – Alternative includes: do nothing
•
Full, informed consent requires that the patient has received and understood five pieces of information: 1. Nature of procedure (What) 2. Purpose or rationale (Why) 3. Risks 4. Benefits 5. Alternatives
FA 2012: 59.3
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FA 2011: 57.2 ME 3e: 32
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FA 2010: 59
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BE03- 3
Exceptions to Informed Consent •
Four exceptions to informed consent: 1. Emergency 2. Waiver by patient 3. Patient is incompetent 4. Therapeutic privilege (unconscious, confused, physician deprives patient of autonomy in interest of health) • Consent can be oral • A signed paper the patient has not read or does not understand does NOT constitute informed consent • Written consent can be revoked orally at any time
FA 2012: 59.4
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FA 2011: 57.3 ME 3e: 32
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FA 2010: 59
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BE03- 4
Emancipated Minors •
Children younger than 18 years are minors and are legally incompetent. – Exceptions: emancipated minors • If older than 13 years and taking care of self (i.e., living alone, treat as an adul). • Marriage makes a child emancipated, as does serving in the military • Pregnancy or giving birth, in most cases, does not • Partial emancipation • Many states have special ages of consent: generally age 14 and older
FA 2012: 59.5
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FA 2011: 57.4 ME 3e: 32
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FA 2010: 59
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BE03- 5
Special Cases for Minor Competency •
Children younger than 18 years are minors and are legally incompetent – Exceptions: • Substance drug treatment • Prenatal care • Sexually transmitted disease treatment • Birth control
FA 2012: 59.5
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FA 2011: 57.4 ME 3e: 32
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FA 2010: 59
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BE03- 6
Patient Confidentiality •
Physicians cannot tell anyone anything about their patient without the patient’s permission. – Physician must strive to ensure that others cannot access patient information. – Getting a consultation is permitted, as the consultant is bound by confidentiality, too. However, watch the location of the consultation. Be careful not to be overheard (e.g., do not discuss in elevator or cafeteria). – If you receive a court subpoena, show up in court but do not divulge information about your patient.
FA 2012: 60.4
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FA 2011: 58.1 ME 3e: 33
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FA 2010: 60
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BE03- 7
Physicians’ Duty to Warn and Protect •
If patient is a threat to self or other, the physician MUST break confidentiality • Duty to warn and duty to protect (e.g.,Tarasoff case + (DWI)) • A specific threat to a specific person (including self) – Tarasoff decision: duty to warn and duty to protect – A student visiting a counselor at a counseling center in California states that he is going to kill someone. When he leaves, the counselor is concerned enough to call the police but takes no further action. The student subsequently kills the person he threatened. The court found the counselor and the center liable because they did not go far enough to warn and protect the potential victim. – The counselor should have called the police and then should also have tried in every way possible to notify the potential victim of the potential danger. In similar situations, first try to detain the person making the threat, next call the police, and finally notify and warn the potential victim. All three actions should be taken, or at least attempted.
FA 2012: 60.5
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FA 2011: 58.2 ME 3e: 33
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FA 2010: 60
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BE03- 8
Malpractice •
Malpractice: – Civil, not criminal • Duty • Breach (dereliction) • Harm done (damage) • Breach caused the harm (direct) – Most common cause of lawsuit is poor communication • Least friendly physicians sued more often
FA 2012: n/a
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FA 2011: 58.3 • ME 3e: 33
FA 2010: 60
•
BE03- 9
Situational Testing: Only 1 Right Answer •
Situational Testing – 1 right answer; many misleading answers – Choose BEST • Dating • Family wants info • Child wants to know more (needs parents permission) • Child pregnant, wants abortion (need consent) • Child pregnant, wants to keep but parents want adoption
! child
wins
• Physician-assisted suicide = no go • Pharmaceutical company bonus = no go, but still use the company • Unneccessary procedures: delve deeper, don’t refer away
FA 2012: 61.1
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FA 2011: 59.1 ME 3e: 30
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FA 2010: 61
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BE03- 10
Development and Physiology
Sam Asgarian, M4 Tulane University
BE04- 1
Elderly Development •
Development and Physiology – Development • Elderly – Sexual changes » Men: longer refractory, slower erection » Women: vaginal dryness – Sleep » Decreased REM, slow-wave, increased wakening – Increased suicide rate – Decreased muscle mass, increased fat – Decreased organ function
FA 2012: 63.1
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FA 2011: 61.1 ME 3e: 29
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FA 2010: 63
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BE04- 2
Grief – Can last 2 months • Pathologic (treat it) if greater than 2 months, excessively strong, or delayed/inhibited/or denied – Kubler-Ross • Denial • Anger • Bargaining • Depression • Acceptance – Not in order, can be overlapping
FA 2012: 63.2
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FA 2011: 61.2 ME 3e: 21
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FA 2010: 63
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BE04- 3
Infants: APGAR Score and Birth Weight •
Apgar score ! pediatrician assesses newborn – 5 areas (each scored 0-2) – APGAR ! Appearance, Pulse, Grimace, Activity, Respiration
•
Birth Weight – Defined as LOW if < 2500g • Increased risk of infection and complication
FA 2012: 62.1
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FA 2011: 60.1 ME 3e: 6
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FA 2010: 61
•
BE04- 4
Milestones: Infant, Toddler, Preschool •
Milestones: – Infant (0-12 months old): reflexes (rooting, startle, etc.) • Sits • Crawls • Walks – Toddler (1-3 yrs old) • Climbs, stacks blocks, talks – Preschool (2.5-4 yrs old) • Toilet trained • Rides tricycle, hops, simple drawings
FA 2012: 62.3
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FA 2011: 60.3 ME 3e: 7
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FA 2010: 62
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BE04- 5
Tanner Development 1. Childhood 2. Pubic hair and breast buds 3. Pubic hair darkens, increased secondary sexual characteristics 4. More development 5. Fully adult
FA 2012: 535.3
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FA 2011: 60.4 ME 3e: 9
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FA 2010: 62
•
BE04- 6
Sexual Dysfunction, Stress, BMI •
Sexual dysfunction – Keep in mind 1) Drugs 2) Diseases 3) Psychological (e.g. fear after MI)
•
Stress – Fatter (lipids, cholesterol, cortisol) – Decreased immune system
•
BMI – Weight over height squared – Know ranges of underweight, normal, overweight, obese, morbidly obese
FA 2012: 63.3, 4, 5
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FA 2010: 63
FA 2011: 61.4, 5, 6 •
•
BE04- 7
ME 3e: 29
Circadian Rhythm and Sleep Circadian rhythm – Suprachiasmatic nucleus of hypothalamus • Prolactin, melatonin, NE
Sleep stages 1) Light sleep 2) Deeper sleep 3 and 4) Deepest sleep (slow wave) 5) REM: dreaming, paralyzed, increased brain O 2, erections • NE reduces REM • Alcohol and other depressants reduce REM and sleep stages 3-4 • ACh is main neurotransmitter in REM sleep
FA 2012: 64.1, 65.2 • FA 2011: 62.1, 63.2 FA 2010: 64 • ME 3e: 10
•
BE04- 8
Narcolepsy and Dreams vs Terrors Narcolepsy: – Excessive daytime sleepiness • Not necessarily asleep, just tired • Tx: give stimulants Dreams are REM, terrors are Stages 3-4 – Night terror ! screaming in middle of night • Common in children • No memory
FA 2012: 65.1, 3
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FA 2011: 63.1, 3 • ME 3e: 12
•
FA 2010: 64
BE04- 9
