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”

+ test

- test

+ disease - disease

TP FP

TNFN

• 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) –or-

1/(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?– 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

- test

13,000

987,000

5,000 995,000

TP FP

FN TN

• 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

+ test

- test

13,000

987,000

5,000 995,000

4,950

50

8,050

986,950

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) In a 75 y/o man should you do a PSA?2) When should PAP smears be initiated?3) What are the live-virus vaccines?4) Who should not receive a live-virus vaccine?5) What patient groups should get the pneumococcal vaccine?6) Is history of chicken pox sufficient to assume immunity and therefore no need to vaccinate?7) Who should get Hepatitis A vaccine?8) True or False: All healthcare workers exposed to a pt. who died of meningoccemia should be

prophylaxed within 48 hrs.9) What are the treatment options for travelers’ diarrhea?10) What is the most effective way to prevent the spread of disease in the hospital? 11) 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?

12) 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?

13) 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?

14) Regarding specificity and sensitivity, which is independent of the prevalence of the disease in a selected population?

15) In what case would the number of false positives be high despite a very high specificity and sensitivity?

16) How is the positive predictive value used in determining whether a screening program is feasible?

17) After what age are mammograms definitely of benefit as a screening test?18) 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?

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

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

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

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!!!!