Statistics & Preventative
Medicine
Board Review
Candice Sech, MD
Statistics
• The difficulty with statistics comes with
all of the jargon
• I will go over the different definitions,
with examples, to help you form a
picture in your mind, and understand
these different concepts
• We will then go over how to interpret
and approach questions likely seen on
the boards
• Sensitivity-Proportion of diseased
population with positive test
– Looks at patients with disease
– Independent of prevalence of disease
– Ex. The sensitivity of a CT scan in detecting
disease X is 97%
– Real words: 97 % of pts. with disease X will
have a positive CT scan (ability to detect when
disease present)
– Formula: TP/TP + FN
• Specificity-Proportion of pts. without
disease with a negative test
– Looks at patients without disease
– Independent of prevalence of disease
– Ex. The specificity of a CT scan in detecting
disease X is 97%
– Real words: 97% of pts. without disease X
will have a negative CT scan (ability to detect
when disease not present)
– Formula: TN/TN + FP
• Positive Predictive Value (PPV)
– Looks at pts. with positive test
– Ex. The PPV of test X for detecting
disease Y is 12%
– Real words: Of pts. with a positive test X,
12% actually have disease Y (true positive)
– Formula: TP/TP + FP
– Does reflect prevalence of disease
• Negative Predictive Value (NPV)
– Looks at pts. with negative test
– Ex. The NPV of test X for detecting disease
Y is 12%
– Real words: Of pts. with a negative test X,
12% actually don’t have disease (True
negative)
– Formula: TN/TN + FN
– Does reflect prevalence of disease
• Use “The table”
+ disease
- disease
+ test
TP
FP
- test
FN
TN
• When prevalence of a disease drops the PPV
falls & NPV rises
– Real words: The less common a disease is, the more
likely that a positive test represents a false positive
– Ex. Pheo is very rare (low prevalence), if you did 24 hr.
urine for metanephrines on everyone, almost all will be
false positives
• When prevalence of a disease increases, the
PPV increases & NPV falls
– Real words: The more common a disease is, the more
likely that a positive test represents a true positive
– Ex. DM is very common (high prevalence), if you tested
everyone for DM, almost all will be true positives
• P Value-significance of a finding
– Usually P values <0.05 are considered
“statistically significant”
– Ex. The P value of high heels causing spurs on
women’s feet is <0.05
– Real words: The likelihood that finding that high
heels cause spurs on women’s feet by chance
alone is less than 5%
– Let’s say in the above example the P value was
<0.5
– Real words: The likelihood that finding that high
heels cause spurs on women’s feet by chance
alone is less than 50%
– That’s a pretty big likelihood that it’s chance alone,
thus not statistically significant
• Number needed to treat (NNT)
– Know this, it will be on your boards
– Real words: How many pts. do I need to treat with
treatment X, to prevent one bad outcome
– Formula: 1/(rate in placebo-rate in treatment group) –or1/(absolute risk reduction)
– Ex. CHF plus drug X-10/50 that received drug died
– CHF plus Placebo-20/50 that received placebo died,
what is the NNT?
• 1/(2/5-1/5) = 1/(.4 - .2) = 1/.2 = 5
• Real words: You must treat 5 pts. with CHF,
with drug X, to prevent one bad outcome
• 95% Confidence Intervals-essentially
same as saying P<0.05
– If the values do not cross zero, it is
considered significant
– Ex. The 95% confidence interval is 0.5 to
1.9, that is considered significant
– If they say the 95% confidence interval is
-0.7 to 1.6 that is non-significant
• Type 1 error-Concluding that there is a
difference (reject null hypothesis) when
there is no difference
• Type 2 error-Concluding that there is no
difference (accept null hypothesis) when
one exists
• They will not give you all of the numbers, and
then let you just calculate sensitivity, etc.
• They may give you some numbers, and then
you figure out the rest, or put it into words,
rather than numbers
• This is why you need to understand the
concepts, rather than just memorizing a
bunch of formulas
• Let’s go over an example……
• Example 1: Incidence of cancer is 1/200 in a
population. For test, sensitivity=99%, and
frequency of abnormal tests in the population is
1.3%, what is the ratio of false positives to true
positives?
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–
–
–
–
If population isn’t given, assume 1 million
An incidence of 1/200, gives 5,000 people with cancer
Abnormal test frequency is 1.3%=13,000 abnl. Tests
Pts. Without cancer=1 million-5000=995,000 pts.
Number of NL tests=1million-13,000=987,000 tests
Now, fill in the table
DZ
No DZ
+ test
TP
FP
- test
FN
TN
5,000
995,000
13,000
987,000
• Fill the rest in
– Sensitivity=TP/(TP+FN)=.99=TP/5,000
– So TP=4,950
– All the others are filled in by subtraction
DZ
No DZ
8,050
+ test
4,950
- test
50
986,950
5,000
995,000
13,000
987,000
Preventative Medicine
• Not many concepts here, unfortunately,
just rote memorization
• Breast Cancer
– Yearly breast exam after age 40
– Yearly mammogram after age 50
– Between age 40-50 use of mammograms is
unclear, some say yearly, some q1-2 years, likely
won’t have question with a pt. in this age range as
screening length is controversial
– Know-High incidence of false positive
mammogram results between ages of 40 to 50
• Blood Pressure/Cholesterol
– BP-every 2 years, and every clinical encounter
– Chol.-Screening for total cholesterol in men 35-65
yrs. old and women 45-65 yrs. old is appropriate,
but not mandatory
• Prostate Cancer
– ACP/ACS recommends PSA be done between ages of
50-69, frequency based on discussion of pluses and
minuses with the pt.
– No PSA recommended >70 yrs. old
• Colon Cancer
– DRE yearly for pts. >40 yrs. old
– FOBT yearly over age of 50
– Sigmoidoscopy-ACS/ACP recommends q3-5 yrs,
starting at age 50
– Colonoscopy recommendations:
• Colonoscopy q10 yrs. after age 50 for average risk pt.
• If polyp is found, repeat in 3 yrs.
• FH of colon cancer screening should begin at age 40, or 10
yrs. prior to age of the family member, the earlier date is
respected
• Follow-up exam in pts. with FH of colon cancer is q5 yrs.
• Multiple family members with colon ca. (Lynch syndrome),
screening begins at age 25, and q1-2 yrs
• Colonoscopy is q1yr. after a hemicolectomy for colon cancer
to verify the absence of recurrence
• Pap Smear
– Start at age 18, or when sexually active
– If three negative results with annual exam,
may continue q3 years (except HIV pts.)
– If previous pap smears have been
negative, patients >70 years old do not
need further smears
• Vaccinations
– Attenuated live virus: MMR, oral polio, nasal influenza,
yellow fever
– Attenuated live bacteria: typhoid (two types) and BCG
– The live vaccines may cause the actual disease in
immunosuppressed patients (remember those with
congenital immunodeficiences)
– All except the attenuated live vaccines can be given in
pregnancy
– AIDS pts.: yearly influenza, hep. B, pneumococcal, HiB,
childhood vaccines (MMR may be given to AIDS pts.)
– Do NOT give AIDS pts. nasal influenza, oral polio, or
smallpox
1) Strep. Pneumoniae vaccine
-Persons older than 2 yrs. of age with
asplenia, SS or an debilitating disease
-Anyone older than 65 yrs. old
-Repeat once in 5-6 yrs.
2) Influenza
-Active within 2 weeks
-Given q1yr. after the age of 50 and also
yearly to high-risk patients, and their
household contacts
-Health care workers
3) Varicella
-All individual older than 12 mths. who aren’t
immune
-Hx. Of chicken pox is sufficient to assume
immunity
4) Hepatitis A
-Persons 2 yrs. of age or older who are at
increased risk of infection by HAV, chronic liver
disease, travelers
5) Hepatitis B
-All those at risk, all adolescents
6) Tetanus
-Booster is recommended q10 yrs.
-May be given at 5 years for “dirty” wound
management
7) Typhoid
-Oral recommended vs. parenteral
8) Yellow Fever
-Based on travel
9) Smallpox
-On demand
-Contraindications: eczema or household contacts
with people with exfoliative skin conditions
-immunosuppression (HIV, steroids >20mg/day)
-radiation therapy
-pregnancy
• Prophylaxis
– Malaria
• Depends on area (chloroquine resistant)
• Malarone, Mefloquine (neuropsychological side
effects), Chloroquine
– Meningococcemia
• Chemoprophylaxis with rifampin, ciprofloxacin,
or ceftriaxone
• Know: Healthcare workers do NOT receive
chemoprophylaxis unless they had recent
“intimate” oral contact with the case patient (ie,
Intubation)
– Travelers’ Diarrhea
• Empiric self treatment v. prevention
• Prevention & treatment: FQs, Bactrim, Azithromycin
• Self treatment:
– 1-2 stools/24 hrs.: none; loperamide
– 3 stools/24 hrs.: Add single dose antibiotic
– 6 stools/24 hrs. & fever or blood: Continue antibiotic x 3d
– Iatrogenic Infections
• Remove lines ASAP
• WASH YOUR HANDS
Questions from Medstudy, 11th edition
1)
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12)
In a 75 y/o man should you do a PSA?
When should PAP smears be initiated?
What are the live-virus vaccines?
Who should not receive a live-virus vaccine?
What patient groups should get the pneumococcal vaccine?
Is history of chicken pox sufficient to assume immunity and therefore no need
to vaccinate?
Who should get Hepatitis A vaccine?
True or False: All healthcare workers exposed to a pt. who died of
meningoccemia should be prophylaxed within 48 hrs.
What are the treatment options for travelers’ diarrhea?
What is the most effective way to prevent the spread of disease in the
hospital?
You have invented a test that is 90% sensitive and 95% specific for screening
of breast cancer. If you tested 100 women with known breast cancer, how
many would the test pick up?
If a study shows new treatment for lung cancer improves survival by 60% and
the P-value is 0.2, would you recommend this treatment?
17)
18)
If a study shows a newer treatment for lung cancer improves survival by 5%
and the 95% confidence interval for the study is 1.6 to 4.9. Would you
consider this new treatment?
Regarding specificity and sensitivity, which is independent of the prevalence
of the disease in a selected population?
In what case would the number of false positives be high despite a very high
specificity and sensitivity?
How is the positive predictive value used in determining whether a screening
program is feasible?
After what age are mammograms definitely of benefit as a screening test?
Are breast self-exams beneficial?
19)
What is the general age group for which pap smears are recommended?
20)
Which are the live vaccines, and which are the dead vaccines? What is their
significance in a pt. who is immunocompromised?
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16)
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PREVENTATIVE MEDICINE PEARLS/SCENARIOS:
DM pts. should be seen by an opthalmologist at the time of diagnosis
Daily ASA should be given to all pts. with increased risk for CAD (>2 risk factors)
Pts. s/p MI with PUD may take daily ASA, with a PPI
If a pt. has concerns re: developing ovarian cancer, has no FH of cancer, there
is no screening test (CA 125 is not done to screen)
Screening CXR are NOT done in pts. with COPD, etc., unless pt. has symptoms
If pt. has grade II esophageal variaces, may begin Nadolol as primary
prophylaxis
KNOW when antibiotic prophylaxis is given for heart lesions and for what
procedures
The only substance known to prevent breast cancer in persons at increased risk
is Tamoxifen
The most important risk factor for the development of colon cancer is age
A 55 year old man, with NO risk factors or symptoms for CAD, does not need a
screening exercise treadmill test, just cholesterol panel
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PREVENTATIVE MEDICINE PEARLS/SCENARIOS
Smoking is a risk factor for pancreatic cancer
A 35 yr. old pt. with Hep. C, genotype 1B, elevated LFTs should receive:
Hep. B, Hep. A, pneumococcal, influenza vaccine
Cervical cancer screening with a PAP smear is primary prevention
A 48 yr. old pt. with DM, LDL-138, HDL-54, should be started on Simvistatin,
LDL goal is 100, treatment started at LDL>130
Pts. with 3 negative pap smears may have q3 paps thereafter
Pts. with HIV and 3 negative pap smears still have paps q1 yrs.
Hand washing is the most effective method of preventing nosocomial
diarrhea in the US
Breast cancer is the most likely cause of death for a woman between the
ages of 45 and 54 in the US
Annual BP measurement has the most evidence to support and is also
recommended by the US Preventative Services Task Force
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PREVENTATIVE MEDICINE PEARLS/SCENARIOS:
A 38 yr. old male with apathy, attitude problems, wide gait, slow
reflexes, anemic, thrombocytopenia, leukopenia, with a high
homocysteine level needs to be considered for B12 deficiency
Deficiency of Folate will increase blood homocysteine level but not
methymalonic acid
A pt. on long term TPN, Chromium deficiency is associated with
diabetes (glucose intolerance)
If a 74 yr. old woman in a NH develops influenza, and the NH residents
haven’t been immunized yet, they should be given rimantidine plus the
influenza vaccine (not effective for 2 weeks)
A 45 yr. old man presents for a vaccine, and has a 6 yr. old child that
just developed chicken pox, he should get varicella immune globulin
Oral polio vaccine can not be given to an immunocomprimised pt.
Low molecular weight heparin starting 12 hrs. post-op is the best
prophylaxis for DVT in a pt. going for hip replacement
Endocarditis prophylaxis is indicated in a pt. with bicuspid valve
undergoing a dental cleaning
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PREVENTATIVE MEDICINE PEARLS/SCENARIOS:
Pts. should be advised to eat at least 6 servings of fruits and/or vegetables
daily
A 32 yr. old obese pt., with no PMH, wanting a pill to lose weight, should be
told that a low calorie diet and exercise are the best ways to lose weight
The half-life of albumin is 3 weeks, and can be used to assess the degree of
malnutrition
Riboflavin deficiency is associated with angular stomatitis, cheilosis,
glossitis, seborrheic dermatitis, and anemia
A pt. with iron deficiency anemia needs to be considered for celiac disease,
and may have an atrophic tongue
A 50 year old female presents to clinic, she only needs a TSH if she has
symptoms of hypothyroidism, not screening TSH
Folic acid helps to prevent certain birth defects
A mammogram should be performed every 1-2 years in women after age 50
In a 55 yr. old with a positive occult stool, both colonoscopy and BE WITH
flex. Sig. are acceptable screening strategies
Resources used: MedStudy, 11th Edition
Conrad Fischer’s Board Review for
Internal Medicine-2005
Thank You!!!!