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EVIDENCE BASED MEDICINE - 2 > 11a – Clinical Diagnosis I > Flashcards

11a – Clinical Diagnosis I Flashcards

1
Q

What are the 2 ways to define normal?

A
  1. Abnormal as unusual
  2. Abnormal as associated with disease
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2
Q

Abnormal as unusual: 2 ‘types’

A
  • Gaussian: mean +/- 2SD
  • Percentile: lower or upper 95%=normal
  • *BOTH assume all diseases have same prevalence
  • *Leads to ‘diagnosis of non-disease’
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3
Q

“Diagnosis of non-disease”

A
  • 95% of normal subjects fall within the reference range for the test
  • 5% of normal do NOT
    o 2.5% on either end
  • *the only normal animals are the ones who haven’t been tested enough yet
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4
Q

Abnormal as associated with disease: 1 ‘type’

A
  • Diagnostic: comparison with a GOLD STANDARD
  • *need clinical judgement
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5
Q

What are most diagnoses based on a combination of? (3)

A
  • Signs
  • Symptoms
  • Tests
  • *ruling in and out disease becomes an assessment of PROBABILITIES
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6
Q

What are the 3 possible actions you will eventually have to choose from (in regards to probability of disease)?

A
  1. Do nothing
  2. Get more info (test or response to treatment)
  3. Treat without obtaining more information
    *Generally depends on PROBABILITY OF DISEASE
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7
Q

What might a diagnostic test include?

A
  • Any technique that differentiates healthy from diseased individuals OR between different diseases
  • Ex. x-rays, stethoscope
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8
Q

What is accuracy?

A
  • Degree of AGREEMENT between ESTIMATED value and the TRUE value
  • QUALITY of test
    o Validity (lack of bias)
    o Reproducibility (precision or repeatability)
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9
Q

What is the ‘equation’ for accuracy?

A
  • Accuracy = validity + reliability
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10
Q

What is validity?

A
  • Ability to measure what it is SUPPOSED to measure without being influenced by other sources of SYSTEMATIC ERRORS
  • *if valid=unbiased
    o Does NO ensure accuracy
  • Not always repeatable
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11
Q

What is reliability?

A
  • Tendency to give the SAME results on REPEATED measures of the same sample
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12
Q

When does a reliable test gives repeatable results?

A
  • Over time
  • Between locations
  • Between populations
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13
Q

What are some sources of false positives and negative results? (KNOW!)

A
  • Laboratory error
  • Improper sample handling
  • Recording errors
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14
Q

What are some laboratory errors?

A
  • Depends on both analytical accuracy and precision
  • Can vary between labs or within labs
  • Does the lab have a recognized quality assurance/control in place?
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15
Q

Doing nasal swaps to detect BRD, importance of timing between sample and culturing

A
  • If more than 3 days after collection=VERY POOR CULTURE RESULTS
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16
Q

What are some sources of false negative results? (KNOW!)

A
  • Improper timing of test
  • Wrong sample
  • Natural or induced tolerance (ex. BVD)
  • Non-specific inhibitors
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17
Q

What are some sources of false positive results? (KNOW!)

A
  • Group cross-reactions
  • Cross contamination (ex. Johne’s disease)
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18
Q

How should the accuracy of any diagnostic test be established?

A
  • ‘BLIND’ comparison to an independent and valid criterion for infection or disease status
  • *GOLD STANDARD
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19
Q

What are examples of diagnostic tests that are accurate (‘gold standard’)?

A
  • CULTURE of organism
  • POST-MORTEM EXAMINATION
  • Biopsy
  • Long-term follow-up
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20
Q

What are pathognomonic tests?

A
  • Absolute predictor of disease or disease agent
  • Can have FALSE negatives
21
Q

What are some examples of a pathognomonic test?

A
  • Culture of MAP (Johne’s disease bacteria)
  • Culture of T. foetus
  • Culture of Salmonella
22
Q

What are surrogate tests?

A
  • Detect SECONDARY changes that will HOPEFULLY predict the presence or absence of disease or the disease agent
  • Can have FALSE NEGATIVES and FALSE POSITIVES
23
Q

What are some examples of surrogate tests?

A
  • Serology
  • Serum chemistry
24
Q

What are the 2 pieces of info when selecting and interpreting diagnostic tests?

A
  • Diagnostic VALIDITY
    o SENSITIVITY
    o SPECIFICITY
  • Need to understand our test subject
    o Prevalence of the disease in source population OR pre-test probability that patient has disease
25
Q

What are the sources of validity?

A
  • Laboratory
  • Test manufacturer
26
Q

What are the sources to understand our test subject?

A
  • Signalment
  • History
  • Clinical examination
  • Published literature
  • Clinical judgement
27
Q

How do you make a 2x2 to use for diagnostic test interpretation?

A
  • Top: Disease present (A, C), disease absent (B, D)
  • Left: Positive (A, B), negative (C, D)
28
Q

What is the cutoff point in the lab?

A
  • Most tests/machines now give us a quantitative value
    o We need to divide them into positives and negatives (tend to OVERLAP)
  • *use the cutoff: ‘positive’ vs. ‘negative’
    o You will get some false positives and false negatives
29
Q

What is sensitivity?

A
  • Proportion of subjects with the disease who have a POSITVE test
  • Indicates how good a test is at detecting disease (positives)
  • =1-false negative
  • SnNout
30
Q

SnNout

A
  • When using tests with VERY HIGH SENSITIVITY, negative results help to RULE-OUT DISEASES
  • *more sensitive=less false negatives
31
Q

What is the equation of sensitivity using a 2x2 table?

A
  • =true positive divided by (true positives + false negatives)
  • *=true positives/all WITH disease
32
Q

What is specificity?

A
  • Proportion of subjects WITHOUT the disease who have a negative test
  • *indicates how good the test is at identifying the non-disease
  • =1-false positive rate
33
Q

SpPin

A
  • When using test with VERY HIGH SPECIFICITY, positive results help to RULE-IN DISEASE
  • *more specificity=less false positives
34
Q

What is the equation of specificity using a 2x2 table?

A
  • =true negatives divided by (true negatives + false positives)
  • *=true negatives divided by all WITHOUT disease
35
Q

What will the cut-off determine?

A
  • SENSITIVITY and SPECIFICITY of the diagnostic test
    o Usually assume them to be CONSTANT
36
Q

How are sensitivity and specificity related?

A
  • INVERSELY RELATED
37
Q

Usually sensitivity and specificity are constant attribute of a diagnostic test, when might they change?

A
  • stage of the disease OR
  • physiological status of the animal
  • Ex. Johne’s disease (slowly progresses over time)
38
Q

Johne’s disease as an example of specificity and sensitivity changing

A
  • Lower serological cutoffs in younger
  • *sensitivity changes
  • Specificity does NOT change
39
Q

What is true prevalence?

A
  • Proportion of population who have the infection under study at one point in time
  • *don’t usually know it
40
Q

What is true prevalence equation from a 2x2 table?

A
  • DISEASE POSTIVE animals (a+c) divided by ALL ANIMALS (n)
41
Q

How do you figure pretest probability?

A
  • Start with disease prevalence
  • Refine to local population
  • Refine to population your serve
  • Refine according to patients presentation
  • Add in results of history and exam
  • Also consider you OWN THRESHOLD FOR TESTING
42
Q

What is apparent prevalence and what is it’s equation?

A
  • *what we normally have (get it from test results)
  • All TEST POSTIVE (a+b) divided by ALL ANIMALS (n)
43
Q

What are the 3 aspects we can use to determine the PREDICTIVE VALUE of our screening test?

A
  1. Sensitivity
  2. Specificity
  3. Prevalence
44
Q

What is positive predictive value? (EXAM)

A
  • More useful in clinic
  • Proportion of patient with positive test results who HAVE the target disorder
  • Affected by sensitivity, specificity, and prevalence
45
Q

What is the positive predictive value (PPV) equation from a 2x2 table?

A
  • PPV = true positives (a) divided by those who TESTED POSITIVE (a+b)
46
Q

What is negative predictive value?

A
  • Proportion with negative test results who DON’T have the target disorder
  • Affected by sensitivity, specific and prevalence
47
Q

What is the negative predictive value (NPV) equation from a 2x2 table?

A
  • NPV = true negatives (d) divided by those who TESTED NEGATIVE (c+d)
48
Q

When looking at the sensitivity and specificity on a 2x2 table, which ‘direction’ are you going?

A
  • GOING DOWN (left: sensitivity) AND UP (right: specificity)
49
Q

When looking at PPV and NPV on a 2x2 table which ‘direction are you going?

A
  • ACROSS THE TABLE (left to right)

EVIDENCE BASED MEDICINE - 2 (3 decks)

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