Why do we perform diagnostic tests?

Reduce uncertainty about the true condition of the patient

- Distinguish between disease and no disease
- Provide prognostic information
- Determine response to therapy
- Provide reassurance

What is an index test?

Why is it used?

The test that we are studying

Always want easier and cheaper tests that give same info as gold standard

What is a gold standard test?

A test generally accepted to be definitive in diagnosis of disease, reference test

Which tests do participants in a diagnosis study receive?

Every patients gets both index test and gold standard test

Why is diagnostic dilemma a criteria for performing a test?

A test is most helpful when it helps reduce uncertainty

Performing a test in a patient whose diagnosis is known is not helpful

Tests are most helpful when they distinguish between 2+ likely diagnoses

What is spectrum bias?

Performance of diagnostic test varies because the people tested are not part of a clinically relevant population

Occurs if a study population does not represent a true diagnostic dilemma

Affects external validity

Draw a 2x2 table for a diagnostic study

What is sensitivity?

How do you calculate sensitivity?

Given the presence of disease, the probability that a test will be positive

Sensitivity = TP / (TP+FN)

(use left column)

What is specificity?

How do you calculate specificity?

Given the absence of disease, the probability that a test will be negative

Specificity = TN / (FP+TN)

(use right column)

What is the formula for sensitivity?

Sensitivity = TP / (TP+FN)

Use left column

What is the formula for specificity?

Specificity = TN / (FP+TN)

Use right column

Is sensitivity used to rule in or out?

SNOUT - sensitivity, rule out

Is specificity used to rule in or out?

SPIN - specificity, rule in

Is sensitivity more useful for screening or confirming a diagnosis?

Sensitivity is useful for screening

Is specificity more useful for screening or confirming a diagnosis?

Specificity is useful for confirming a diagnosis

Increased sensitivity is related to [direction, sign] predictive value

Increased sensitivity is related to increased negative predictive value

Increased specificity is related to [direction, sign] predictive value

Increased specificity is related to increased positive predictive value

Describe the important points about sensitivity

Given the presence of disease, the probability that a test will be positive

Minimize false negatives

SNOUT - sensitivity, rule out

Most useful when test is negative

Increased negative predictive value

Screening

Sensitivity = TP / (TP+FN)

(use left column)

Describe the important points about specificity

Given the absence of disease, the probability that a test will be negative

Minimize false positives

SPIN - specificity, rule in

Most useful when test is positive

Increased positive predictive valuie

Confirming a diagnosis

Specificity = TN / (FP+TN)

(use right column)

What is the likelihood ratio?

What advantages does it have over sensitivity and specificity?

What is it used to calculate?

What is the formula for likelihood ratio?

Another way to describe the performance of a test

Can be used across a spectrum of test

Used to calculate post-test probability of a disease

LR = P(result in diseased person) / P(result in non-diseased person)

= true rate / false rate

What is the formula for positive likelihood ratio?

LR+ = P(positive result in diseased person) / P( positive result in non-diseased person)

= true positive rate / false positive rate

= [TP/(TP+FN)] / [FP/(FP+TN)]

= sensitivity / (1 – specificity)

What is the formula for negative likelihood ratio?

LR- = P(negative result in diseased person) / P(negative result in non-diseased person)

= false negative rate / true negative rate

= [FN/(TP+FN)] / [TN/(FP+TN)]

= (1 – sensitivity) / specificity

In calculating a likelihood ratio, sensitivity is in the _____ and specificity is in the _____

In calculating LR,

- sensitivity is in the numerator
- specificity is in the denominator

Negative LR is influenced by ______

Sensitivity

(important to rule out disease)

Positive LR is influenced by ______

Specificity

(important to rule in diseasea)

What is the best negative LR?

0

What is the best positive LR?

Infinity

What is a completely unhelpful LR?

1

What is the relationship between pre-test odds, LR, and post-test odds?

Pre-test odds x LR = Post-test Odds

How can you convert probability into odds?

Odds = probability / (1 – probability)