Screening Flashcards
sensitivity
Definition: The ability of a test to correctly identify those with the disease (true positive rate).
= TP / (TP+FN)
Interpretation: A highly sensitive test minimizes false negatives, making it useful for ruling out a disease (SnNout: a highly Sensitive test, when Negative, rules OUT the disease).
specificity
Definition: The ability of a test to correctly identify those without the disease (true negative rate).
Formula:
= TN / (TN + FP)
Interpretation: A highly specific test minimizes false positives, making it useful for confirming a disease (SpPin: a highly Specific test, when Positive, rules IN the disease)
PPV
Definition: The probability that a person who tests positive actually has the disease.
Formula:
= TP / (TP + FP)
Interpretation: PPV depends on disease prevalence. Higher prevalence increases PPV, meaning a positive result is more likely to be a true positive.
NPV
Definition: The probability that a person who tests negative actually does not have the disease.
Formula:
= TN / (TN + FN)
Interpretation: Like PPV, NPV depends on prevalence. Higher prevalence decreases NPV, meaning a negative result is less reliable for ruling out disease.
Likelihood ratios
Likelihood Ratios: Definition, Formula, and Interpretation
Definition
A likelihood ratio (LR) is a measure used in statistics and diagnostic testing to assess how much a test result changes the probability of a condition being present. It compares the probability of obtaining a given test result in people with a condition versus those without it.
Formula
The likelihood ratio is calculated differently depending on whether the test result is positive or negative:
1. Positive Likelihood Ratio (LR⁺):

= sensitivity / 1-specificity
This measures how much more likely a positive test result is in someone with the disease compared to someone without it.
2. Negative Likelihood Ratio (LR⁻):

=1-sensitivity / specificity
This measures how much less likely a negative test result is in someone with the disease compared to someone without it.
Interpretation
* LR⁺ values:
* >10: Strong evidence to rule in a disease
* 5–10: Moderate evidence to rule in
* 2–5: Weak evidence
* 1: No diagnostic value
* LR⁻ values:
* <0.1: Strong evidence to rule out a disease
* 0.1–0.2: Moderate evidence
* 0.2–0.5: Weak evidence
* 1: No diagnostic value
Likelihood ratios can be used with Bayes’ theorem to update pre-test probabilities and obtain post-test probabilities, helping clinicians and researchers make informed decisions.
stages of a screening programme implementation
identifying and inviting participants
informing participants
selecting screening test
setting the threshold
diagnostic phase
treatment phase
evaluation
wilson junger criteria
Disease:
- it is important
- do we understand the natural history
- is there a long enough phase between pre-clinical stage and disease
- does early detection improve outcomes
Test:
- validity
- acceptability
- reliability and affordability
Diagnosis and treatment:
- are diagnosis facilities available and accessible - wait times
- is treatment available
- is the treatment cost effective and sustainable
- do benefits outweigh harm
other
- ethics
- inequalities
- governance
– legal and economic factors
Length time bias
tendency to detect less aggressive forms of disease with better outcomes which have better outcomes. Aggressive forms have a shorter pre-clinical phase.
Lead time bias
People who’s disease is detected early through screening appear to survive longer than those who’s disease was detected later but actual survival time with disease didn’t vary, it was just diagnosed earlier.
Pre natal screening programmes
Downs syndrome
Sickle Cell and thalassaemia
11 physical conditions at 20 week scan
infectious diseases - HIV, hepatitis
newborn screening
heel prick test / blood spot test for 9 rare diseases
sight and hearing
physical exam - heart, hips, testes
diabetic retinopathy screening
annual
people with diabetes over age 12
cervical cancer screening
women aged 25 to 49 - every three years
50-64 - every five years
Breast cancer
women every 3 years between
ages of 50 and 71
Bowel cancer screening
everyone 60 to 74
every 2 years
Abdominal aortic aneurysm (AAA) screening
Men aged 65
Target lung health checks
everyone aged between 55 and 74 who either smoke or used to smoke
Chlamydia screening (non NHS scheme)
all sexually active women under the age of 25 accessing a sexual and reproductive health service should be offered a test
NHS health check
tests for risk of heart disease, diabetes, stroke, kidney disease
40-74 year olds
child measurement programme
height and weight of all children
aged 4/5 years and age 10/11
targeted TB screening (generally local)
chest x ray
people at high risk but asymptomatic - ports of entry, prisons, homeless shelters
