Tutorial 2a

Sujets

• Testudo
• Tutoriel
• Disorder
• Influence (politique)
• Scrutin proportionnel plurinominal
• LRS

Informations

• Publié par: Sauf
• Nombre de lectures: 85
• Langue: English

Extrait

Tutorial 2a

Tutorial 2

How to evaluate an article about a diagnostic test

Step 1: Are the results valid?

When looking at the validity of the study, your goal is to determine if you can believe the results of the

study and if the reported diagnostic accuracy is true.

Step 1a: Was there an independent, blind comparison with a reference standard?

Those interpreting the results of the new diagnostic test should not be aware of the results

of the reference test. Knowing the results of one study influences how well subsequent

studies perform. Seeing the results of a CT scan may influence your ability to pick up a

AAA on a subsequent physical exam.

In order to determine the accuracy of a test it must be compared with the "gold standard"

for determining the diagnosis (biopsy, autopsy, long term follow-up or a widely accepted

test). If you compared a new blood test for the diagnosis of prostate cancer to PSA then

you would not know the accuracy of this test for the diagnosis of prostate cancer (biopsy is

the gold standard) but only how it compares to PSA testing.

Step 1b: Did the patient sample include an appropriate spectrum of patients?

A given test is only valuable if it can distinguish between having and not having a disorder

in the same population in which the test would be clinically useful. A blood test shown to

be positive in 80% of a population of patients with ST elevation MI may not be useful in a

general population of patients with chest pain. A more useful study might look at the blood

test in patients suspected of having MI.

Step 1c: Did the test results influence the decision to perform the reference standard?

The characteristics of the study test will change if the results influence whether or not a

confirmatory test is performed. This "verification bias" can be problematic when the

reference standard is invasive or harmful and clinicians are reluctant to have patients

undergo a potentially harmful test. If only patients with a positive screening test for PE

underwent pulmonary angiography then you would not know how many PE's were missed

in the group that did not have angiography.

Step 1d: Were the methods for performing the test described in sufficient detail?

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You must be able to replicate the exact test performed in the study in order to apply it to

your patients. The technique for analysis and interpretation must be described especially

with more subjective tests.

Step 2: What are the results?

Step 2a: What is the likelihood ratio (LR)

The LR expresses the accuracy with which a diagnostic test identifies a disorder. The LR

should be calculated for each level of a result if the test does not give a yes/no or

positive/negative answer (such as with VQ scans). The LR tells you how likely a given test

result is in patients with a disease compared with how likely the test result is in patients

without the disease. When calculating the LR (whether positive or negative) the proportion

of patients with disease will always be in the numerator and the proportion without the

disease will be in the denominator. A LR of 10 tells you that a given test result is 10 times

more likely to occur in a patient with as opposed to without a disease.

Example of LR from the article on D-Dimer sensitivity and specificity:

D-dimer result

PE present

PE absent

positive

167

310

negative

30

670

total

197

980

LR+ = positive test in those with disease / positive test in those without disease

LR+ = (167/197) / (310/980) = sensitivity / (1-specificity) = 2.7

LR- = negative test in those with disease / negative test in those without disease

LR- = (30/197) / (670/980) = (1-sensitivity) / specificity = 0.22

How useful a LR is depends on how much it will raise or lower the pretest probability of

the disorder. LRs > 1 increase the liklihood of a disorder and LRs < 1 decrease the

liklihood of a disorder. In general, LR's less than or equal to 1 and LR's greater than 10

will be robust enough to significantly change the pretest probability. The easiest way to

convert pretest probability to post-test probability is to use the Fagan nomogram (

click

here to use an interactive nomogram

). Otherwise you need to convert the pre-test

probability to odds (probability/[1-probability]) and multiply by the LR to get the post-test

odds. The post-test odds can then be converted to probability (odds/[odds+1]). Using LRs

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obviates the need for using sensitivity and specificity.

Step 2b: Sensitivity and Specificity

Although the literature is moving towards reporting LRs, older studies will still put results

in terms of sensitivity and specificity. Sensitivity is the proportion of patients with a

disorder who have a positive test, and specificity is the proportion of patients without a

disorder who have a negative test. A test with high sensitivity is useful for ruling out

(Snout = "sensitivity rules out") a disorder and a test with a high specificity is useful for

ruling in (Spin = "specificity rules in") a disorder. These terms are more limiting than LRs

because test results must be dichotomous or able to fit in a 2x2 table in order to calculate

sensitivity and specificity. A good example of these limitations comes from the Pioped

study in which the authors had to compress the results of 4 possible VQ scans (high prob,

intermediate prob, low prob, and normal) into either positive or negative in order to

calculate the sensitivity and specificity. LRs would have allowed them to keep each test

result separate and calculate the likelihood of PE for any given VQ scan result.

Example of sensitivity and specificity from the above table:

Sensitivity = proportion of patients with disease who have a positive test

Sensitivity = 167 / (167+30) = 167/197 = 85%

Specificity = proportion of patients without disease who have a negative test

Specificity = 670 / (310+670) = 670/980 = 68%

Step 3: Will the results help in caring for my patients?

A test must be reproducible in your clinical setting in order to be useful. If the test requires

highly skilled individuals to interpret or observer disagreement is high, the test may be less

useful outside of the study setting. Test properties can change in different sub populations

with more or less severe disease or in the presence of other conditions that influence the

test result. You must compare your population of patients with the study population and

examine the inclusion and exclusion criteria. If your patients are similar enough to the

study population then you need to consider if the test result is robust enough to move your

pre-test probability to a post-test probability that would alter treatment. In other words,

would the test result influence the management of an individual patient?

References

1. Jaeschke R, Guyatt G, Sackett DL. Users' guides to the medical literature, III: how to use an article

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about a diagnostic test, A: are the results of the study valid? JAMA. 1994;271:389-391.

2. Jaeschke R, Guyatt G, Sackett DL. Users' guides to the medical literature, III: how to use an article

about a diagnostic test, B: what are the results and will they help me in caring for my patients? JAMA.

1994;271:703-707.

3. Dawson-Saunders B, Trapp RG. Basic and Clinical Biostatistics. Norwalk, CT: Appleton and Lang ,

1994.

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Tutorial 2a

Article:

Sensitivity and Specificity of a Rapid Whole-Blood Assay for D-Dimer in the Diagnosis of

Pulmonary Embolism

Authors:

Ginsberg JS, Wells PS, Kearon C, et al

Publication:

Ann Intern Med. 1998;129:1006-1011

Study:

Prospective cohort of 1177 consecutive patients with suspected PE underwent whole-blood D-

dimer assay compared with diagnostic strategy for diagnosis of PE

Inclusion Criteria

: consecutive patients 18 years or older with clinically suspected PE

Exclusion Criteria:

suspected upper extremity DVT; no symptoms within 48 hours of presentation;

treatment with anticoagulants for greater than 72 hours; life expectancy < 3 months; contraindication to

contrast dye; geographic inaccessibility

Step 1: Are the results valid?

Step 1a: Was there an independent, blind comparison to a reference standard?

Yes, but did not use pulmonary angiography as reference standard and instead used a diagnostic strategy

in which all patients had a VQ scan and US. Patients at highest risk underwent venography and

pulmonary angiography if venography was negative. All patients with negative test results were followed

for 3 months.

Step 1b: Did patient sample include appropriate spectrum of patients?

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Yes, all patients suspected of having PE were included regardless of low, moderate, or high pre-test

probability.

Step 1c: Did results influence the decision to perform reference standard?

No

Step 1d: Were methods of performing the test described in sufficient detail?

Yes

Step 2: What are the results?

D-dimer result

PE present

PE absent

LR (95% CI)

positive

167

310

2.7 ( 2.3-3.0)

negative

30

670

0.22 (0.16-0.31)

total

197

980

Patient

Subgroup

PE

present

PE

present

PE absent PE absent

LR

(95% CI)

LR

(95% CI)

+ D-

dimer

- D-dimer

+ D-

dimer

- D-dimer + D-dimer - D-dimer

low

pretest

probability

19

5

163

516

3.3

(2.6-4.2)

0.27

(0.13-0.60)

moderate

pretest

probability

81

20

136

145

1.7

(1.4-1.9)

0.38

(0.26-0.58)

high

pretest

probability

67

5

11

9

1.7

(1.1-2.5)

0.15

(0.06-0.41)

Step 3: Will the results help me in caring for my patients?

Limitations

1. used complex diagnostic strategy instead of pulmonary angiography as "gold standard"

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2. valid for whole-blood D-dimer by simpliRED assay with results reported as normal or abnormal

Strength

1. included data on pre-test probability

Conclusion

In patients with suspected PE, a normal D-dimer is useful for excluding the diagnosis in those with a low

pre-test probability. A normal D-dimer does not exclude PE in pateints with moderate to high pretest

probability

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