L5B - Diagnostic Screening and Testing


  1. Define screening including goals and risks.
  2. Describe how the validity of diagnostic evidence is assessed.
  3. Explain the basic statistical measures of validity of diagnostic & screening tests (sensitivity, specificity, positive and negative predictive values, likelihood ratios)
  4. Discuss the application of the results to patient care.

Screening Test

A test administered go a group of asymptomatic people to detect signs of disease


Used as a secondary prevention technique

  • Improve prognosis with early diagnosis
  • Reduce mortality
  • Reduce severity of disease

Are the results Valid?


Casp offers a checklist for each type of paper


  1. https://casp-uk.net/wp-content/uploads/2018/01/CASP-Diagnostic-Checklist.pdf

  2. https://casp-uk.net/wp-content/uploads/2018/03/CASP-Diagnostic-Checklist-2018_fillable_form.pdf

Measuring Diagnostic Accuracy


Important for planing therapy, if reliable and accurate, result can be acted o with a high degree of confidence


  1. Sensitivity ad Specificity is NOT affected by prevalence of the disease
  2. PPV IS dependent on the prevalence o the disease in population

Goal of Population Screening

Identify

  1. a pre-disease abnormality
  2. early disease
  3. disease risk markers

Screening is intended for all people, in an identified target population, who
do not have symptoms of the disease or condition being screened for.

Risks of Screening

  1. False Positives
  2. False Negatives
  3. Over-Diagnosis

iShould domore good than harm

Diagnostic Test

identifying whether a person has or does not
have a particular condition.


Best Study Design

  • *Cross-sectional survey prospective,
    blind comparison to a gold standard*
  • *systematic reviews are better than an individual
    study.*

Introducing New Diagnosis

To achieve this, researchers should give all participants
an index text (new test) and a reference test (currently
best test at diagnosing the condition; the gold standard)
• The results of the index test and the reference test are
compared.
• The number of patients who tested true positive (TP),
true negative (TN), false positive (FP) and false negative
(FN).


The sensitivity and specificity of the index test can then
be calculated.

Sensitivity
• measures the proportion of patients with the disease who also test positive
for the disease in this study. It is the probability that a person with the
disease will have a positive test result.

Sensitivity
• measures the proportion of patients with the disease who also test positive
for the disease in this study. It is the probability that a person with the
disease will have a positive test result.

Assessing The Validity of Diagnostic Tests

1. Was there a clear question for the study to address

  1. *The patients should include patients with a
    high, medium and low probability of having the target disease.*
  2. usefulness of a test is demonstrated in its ability to distinguish between obvious illness and those cases where it is not so obvious

2. Is the Comparison with an appropriate reference standard

  • The reference (or gold) standard refers to the commonly accepted proof that the target disorder is present or not present.

3. Did all participants get the diagnostic test and the reference standard?

If not, you will likely have verification bias (missing data)
• Common when the reference test is harmful or invasive.

4. Could the results of the test of interest have been influenced by the results of the reference standard, or vice versa?

  • Blind study is best (unable to influence if blinded) – the results both tests should each be decided without knowledge of the results of the other test.

5. Was there a clear description of the disease/condition status of the tested population?

6. Was there sufficient description of the methods for performing the test?

Was this Test Valid?

Sensitivity

true positive / all diseased individuals


  • TP/(TP + FN)


  • High Sensitivity High positive accuracy



RELATES TO TEST

Specificity

true negative / all non-diseased individuals


  • TN/(TN + FP)
  • High Specificity = High negatve accuracy

RELATES TO TEST

Positive Predictive Value (PPV)

  • Probability that the patient has the disease if they tested positive
  • True Positive/(True pos + False Pos)

RELATES TO INDIVIDUAL

Negatve Predictive Value

  • The probability that the patient doesn't have the disease if they tested negative
  • True Negative/(True Neg + False Neg)

RELATES TO INDIVIDUAL

Consequences of Low Sensistivty

  • People with the disease are missed (false
    negatives)
  • False reassurance that people do not have
    disease
  • People might delay seeking treatment
  • Costs without the benefits*

Consequences of Low Specificity

  • People who do not have the disease may believe that they do (False positives)
  • Unnecessary anxiety
  • Unnecessary investigative procedures
  • Costs will increase with no compensating benefits

Likelihood Ratio

Likelihood Ratois can be used to help adapt the results of a study to a patient, by determining the probability of disease in the patients

LR's

  1. Alternative method of assessing the performance of a diagnostic test
  2. Expres how many times a test result will be found in diseased, compared wth non-diseased people
  3. Indicate the LIKELIHOOD that we can expect a certain result (pos/neg) in a patient with the disorder, compared with the LIKELIHOOD that that result would also be expected from a patient without that disorder

Equations

  • LR Positive = Sensitivity/(1-specificity)
  • LR negative = 1 - (sensitivty)/(specificity)

Intrepreting LR's

  • For a postive test, HIGH LRs indicate that the test is effective at diagnosing disease
  • For a negative test, LOW LR's indicate that the test is effective at diagnosing non-disease