Updated: 1/24/2019

2x2 Tables (SN, SP, PPV, NPV, OR, RR)

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Sensitivity, Specificity, PPV, NPV
  • 2x2 Table Sensitivity and SpecificityThese 4 measures describe how well diagnostic tests capture the true presence or absence of disease 
  • Sensitivity (SN)
    • % with disease who test positive
    • = a/(a+c) = TP/(TP+FN)
  • Specificity (SP)
    • % without disease who test negative
    • = d/(b+d) = TN/(FP+TN)
  • Positive predictive value (PPV)     
    • % positive test results that are true positives
    • = a/(a+b) = TP/(TP+FP)
  • Negative predictive value (NPV) 
    • % negative test results that are true negatives
    • = d/(c+d) = TN/(FN+TN)
  • Cut-off point may be adjusted to optimize sensitivity and specificity, which are inversely related (cut-off point with decreased sensitivity is associated with increased specificity and vice-versa)  
  • Receiver operating characteristic (ROC) curves are a graphical depiction of a test's performance
    • Y axis: sensitivity
    • X axis: 1-specificity
    • The higher the curve, the better the test
    • This is quantified by the AUC (area under the curve); an AUC of 0.5 states that the test performs no better than chance (bad test!), whereas an AUC of 0.9 suggests a better-performing test
Odds Ratio, Relative Risk, Attributable Risk, Absolute Risk Reduction
  • 2x2 TableThese measures describe the relationship between a risk factor and a disease
  • Odds Ratio (OR)
    • odds of having disease in expose group / odds of having disease in unexposed group 
      • = ad/bc
  • Relative Risk (RR)  
    • probablity of getting disease in exposed group / probability of getting disease in unexposed group
      • = [a/(a+b)] / [c/(c+d)]
  • Attributable Risk (AR) 
    • risk in exposed group - risk in unexposed group
      • = a/(a+b) - c/(c+d)
  • Absolute Risk Reduction (ARR) 
    • risk in control group - risk in intervention group
      • = c/(c+d) - a/(a+b)
 
 

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Questions (16)
Lab Values
Blood, Plasma, Serum Reference Range
ALT 8-20 U/L
Amylase, serum 25-125 U/L
AST 8-20 U/L
Bilirubin, serum (adult) Total // Direct 0.1-1.0 mg/dL // 0.0-0.3 mg/dL
Calcium, serum (Ca2+) 8.4-10.2 mg/dL
Cholesterol, serum Rec: < 200 mg/dL
Cortisol, serum 0800 h: 5-23 μg/dL //1600 h:
3-15 μg/dL
2000 h: ≤ 50% of 0800 h
Creatine kinase, serum Male: 25-90 U/L
Female: 10-70 U/L
Creatinine, serum 0.6-1.2 mg/dL
Electrolytes, serum  
Sodium (Na+) 136-145 mEq/L
Chloride (Cl-) 95-105 mEq/L
Potassium (K+) 3.5-5.0 mEq/L
Bicarbonate (HCO3-) 22-28 mEq/L
Magnesium (Mg2+) 1.5-2.0 mEq/L
Estriol, total, serum (in pregnancy)  
24-28 wks // 32-36 wks 30-170 ng/mL // 60-280 ng/mL
28-32 wk // 36-40 wks 40-220 ng/mL // 80-350 ng/mL
Ferritin, serum Male: 15-200 ng/mL
Female: 12-150 ng/mL
Follicle-stimulating hormone, serum/plasma Male: 4-25 mIU/mL
Female: premenopause: 4-30 mIU/mL
midcycle peak: 10-90 mIU/mL
postmenopause: 40-250
pH 7.35-7.45
PCO2 33-45 mmHg
PO2 75-105 mmHg
Glucose, serum Fasting: 70-110 mg/dL
2-h postprandial:<120 mg/dL
Growth hormone - arginine stimulation Fasting: <5 ng/mL
Provocative stimuli: > 7ng/mL
Immunoglobulins, serum  
IgA 76-390 mg/dL
IgE 0-380 IU/mL
IgG 650-1500 mg/dL
IgM 40-345 mg/dL
Iron 50-170 μg/dL
Lactate dehydrogenase, serum 45-90 U/L
Luteinizing hormone, serum/plasma Male: 6-23 mIU/mL
Female: follicular phase: 5-30 mIU/mL
midcycle: 75-150 mIU/mL
postmenopause 30-200 mIU/mL
Osmolality, serum 275-295 mOsmol/kd H2O
Parathyroid hormone, serume, N-terminal 230-630 pg/mL
Phosphatase (alkaline), serum (p-NPP at 30° C) 20-70 U/L
Phosphorus (inorganic), serum 3.0-4.5 mg/dL
Prolactin, serum (hPRL) < 20 ng/mL
Proteins, serum  
Total (recumbent) 6.0-7.8 g/dL
Albumin 3.5-5.5 g/dL
Globulin 2.3-3.5 g/dL
Thyroid-stimulating hormone, serum or plasma .5-5.0 μU/mL
Thyroidal iodine (123I) uptake 8%-30% of administered dose/24h
Thyroxine (T4), serum 5-12 μg/dL
Triglycerides, serum 35-160 mg/dL
Triiodothyronine (T3), serum (RIA) 115-190 ng/dL
Triiodothyronine (T3) resin uptake 25%-35%
Urea nitrogen, serum 7-18 mg/dL
Uric acid, serum 3.0-8.2 mg/dL
Hematologic Reference Range
Bleeding time 2-7 minutes
Erythrocyte count Male: 4.3-5.9 million/mm3
Female: 3.5-5.5 million mm3
Erythrocyte sedimentation rate (Westergren) Male: 0-15 mm/h
Female: 0-20 mm/h
Hematocrit Male: 41%-53%
Female: 36%-46%
Hemoglobin A1c ≤ 6 %
Hemoglobin, blood Male: 13.5-17.5 g/dL
Female: 12.0-16.0 g/dL
Hemoglobin, plasma 1-4 mg/dL
Leukocyte count and differential  
Leukocyte count 4,500-11,000/mm3
Segmented neutrophils 54%-62%
Bands 3%-5%
Eosinophils 1%-3%
Basophils 0%-0.75%
Lymphocytes 25%-33%
Monocytes 3%-7%
Mean corpuscular hemoglobin 25.4-34.6 pg/cell
Mean corpuscular hemoglobin concentration 31%-36% Hb/cell
Mean corpuscular volume 80-100 μm3
Partial thromboplastin time (activated) 25-40 seconds
Platelet count 150,000-400,000/mm3
Prothrombin time 11-15 seconds
Reticulocyte count 0.5%-1.5% of red cells
Thrombin time < 2 seconds deviation from control
Volume  
Plasma Male: 25-43 mL/kg
Female: 28-45 mL/kg
Red cell Male: 20-36 mL/kg
Female: 19-31 mL/kg
Cerebrospinal Fluid Reference Range
Cell count 0-5/mm3
Chloride 118-132 mEq/L
Gamma globulin 3%-12% total proteins
Glucose 40-70 mg/dL
Pressure 70-180 mm H2O
Proteins, total < 40 mg/dL
Sweat Reference Range
Chloride 0-35 mmol/L
Urine  
Calcium 100-300 mg/24 h
Chloride Varies with intake
Creatinine clearance Male: 97-137 mL/min
Female: 88-128 mL/min
Estriol, total (in pregnancy)  
30 wks 6-18 mg/24 h
35 wks 9-28 mg/24 h
40 wks 13-42 mg/24 h
17-Hydroxycorticosteroids Male: 3.0-10.0 mg/24 h
Female: 2.0-8.0 mg/24 h
17-Ketosteroids, total Male: 8-20 mg/24 h
Female: 6-15 mg/24 h
Osmolality 50-1400 mOsmol/kg H2O
Oxalate 8-40 μg/mL
Potassium Varies with diet
Proteins, total < 150 mg/24 h
Sodium Varies with diet
Uric acid Varies with diet
Body Mass Index (BMI) Adult: 19-25 kg/m2
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(M1.ST.10) A new real time-PCR test for the hepatitis C virus is approved for medical use. The manufacturer sets the threshold number of DNA copies required to achieve a positive result such that the sensitivity is 98% and the specificity is 80%. The tested population has a hepatitis C prevalence of 0.7%. Which of the following changes in the prevalence, incidence, or threshold concentration will increase the positive predictive value of the test, if the other two values are held constant? Review Topic

QID: 103972
1

An increase in incidence

8%

(2/25)

2

An increase in prevalence

56%

(14/25)

3

A decrease in incidence

4%

(1/25)

4

A decrease in prevalence

0%

(0/25)

5

Lowering the threshold concentration required for a positive test.

12%

(3/25)

M1

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(M1.ST.25) An at-home recreational drug screening test kit is currently being developed. They consult you for assistance with determining an ideal cut-off point for the level of the serum marker in the test kit. This cut-off point will determine what level of serum marker is associated with a positive or negative test, with serum marker levels greater than the cut-off point indicative of a positive test and vice-versa. The cut-off level is initially set at 4 mg/uL, which is associated with a sensitivity of 92% and a specificity of 97%. How will the sensitivity and specificity of the test change if the cut-off level is raised to 6 mg/uL? Review Topic

QID: 105635
1

Sensitivity decreases, specificity decreases

0%

(0/32)

2

Sensitivity increases, specificity decreases

16%

(5/32)

3

Sensitivity decreases, specificity increases

56%

(18/32)

4

Sensitivity increases, specificity increases

9%

(3/32)

5

Sensitivity decreases, specificity may increase or decrease

3%

(1/32)

M1

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(M1.ST.6) A 16-year-old female is seen at her outpatient primary medical doctor's office complaining of a sore throat. Further history reveals that she has no cough and physical exam is notable for tonsillar exudates. Vitals in the office reveal HR 88, RR 16, and T 102.1. Using the Centor criteria for determining likelihood of Group A beta-hemolytic strep pharyngitis, the patient has a score of 3. A review of the primary literature yields the findings in Image A. What is the specificity of the Centor criteria using a score of 3 as a cutoff value? Review Topic

QID: 103968
FIGURES:
1

9/54 = 17%

7%

(2/27)

2

41/50 = 82%

52%

(14/27)

3

41/46 = 89%

11%

(3/27)

4

45/50 = 90%

0%

(0/27)

5

Not enough information has been provided

4%

(1/27)

M1

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(M1.ST.4) Health officials are considering a change be made to the interpretation of the tuberculin skin test that will change the cut-off for a positive purified protein derivative (PPD) from 10 mm to 5 mm for healthcare workers. Which of the following is a true statement regarding this potential change? Review Topic

QID: 103966
1

This change will decrease the positive predictive value of the test

44%

(8/18)

2

This change will decrease the negative predictive value of the test

22%

(4/18)

3

This change will not change the negative predictive value of the test

6%

(1/18)

4

This change will not change the sensitivity of the test

6%

(1/18)

5

This change will increase the specificity of the test

6%

(1/18)

M1

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(M1.ST.1) As a public health practitioner, you are charged with choosing which blood test to use for the screening of a new infectious disease. You want to ensure that as many cases of this disease as possible are picked up by this initial screening test. Which of the following is the highest priority characteristic of this new test? Review Topic

QID: 103963
1

Highly sensitive test

79%

(15/19)

2

Highly specific test

0%

(0/19)

3

Low false positive rate

0%

(0/19)

4

High true positive rate

0%

(0/19)

5

Low negative predictive value

0%

(0/19)

M1

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