ScreeningPHIL THIRKELL

What is screening?

A process of identifying apparently healthy people who may be at risk of a disease or condition

Identify

Apparently healthy

Increased risk of a disease/condition

Give 4 screening programmes undertaken in the UK.

Antenatal screening

Postnatal screening – hearing, heel prick, neuroblastoma

Cervical smear

Mammography

Chlamydia screening

Bowel Cancer – FOBT

Prostate cancer

Abdominal Aortic Aneurysm

Depression – PHQ-9 questionnaire

etc.

Criteria for a Screening Programme Wilson + Jungner criteria

1. Important health problem

2. Treatment available

3. Facilities available for diagnosis and treatment

4. Latent stage of the condition

5. Test available to detect the condition

6. Test is acceptable to the population

7. Natural history of the disease is known

8. Policy of who gets treatment has been made

9. Financially viable

10. Case-finding is a continual process, not just a one off

Neonatal screening

Which conditions are screened for with blood spot testing?

Phenylketonuria

Sickle cell disease

Cystic fibrosis

Congenital hypothyroidism

Medium-chain acyl-CoA dehydrogenase deficiency

Antenatal Screening

What is a pregnant woman screened for?

Pre-eclampsia

Rhesus antigen status / blood group

Anaemia

Diabetes

Syphilis

Hepatitis B/C

HIV

Anomaly Scan – USS between 18-21 weeks

What is an anomaly scan used for? Spina bifida

Down’s syndrome

Hydrocephalus

Cleft lip/palate

Date the pregnancy

Sex of the baby

Multiple pregnancy

Organ development

Abdominal wall

Test Outcomes

Diseased Non-Diseased

Test positive True Positive False Positive

Test negative False Negative True Negative

Sensitivity

The number of people who have the disease who get a positive test result

True positive / (True positive + False Negative)

e.g. 50 people with known Rheumatoid Arthritis. RhF blood test is positive in 42 of the patients.

Sensitivity is 84%

Specificity

The number of people who don’t have a disease who are correctly told they don’t have it

True negatives / (True negatives + False positives)

E.g. 30 patients with no evidence of rheumatoid arthritis have a blood test for RhF. 2 patients have a positive result.

Specificity = 93%

Positive Predictive Value

The number of people who have a positive test result who actually do have the disease

True positives / (True positives/False positives)

e.g. 2500 PSA blood tests performed on men >65yr. 800 are raised above normal levels. Biopsy reveals that 95 of these have prostate cancer.

PPV = 95/(95+800) = 11%

Negative Predictive Value

The number of people who have a negative test result who definitely don’t have the disease

True negatives / (true negatives + false negatives)

e.g. 2500 PSA blood tests on men >65yrs. 1700 have normal PSA results. 20 of these turn out to currently have prostate cancer despite a normal PSA.

1680/ (20 + 1680) = 98.8%

Diseased Non-Diseased

Test positive True Positive False Positive

Positive Predictive Value

TP / (TP + FP)

Test negative False Negative True Negative

Negative Predictive Value

TN / (TN + FN)

Sensitivity

TP / (TP + FN)

Specificity

TN / (TN + FP)

Screening Bias

Healthy screenee

Length time

Lead time

Overdiagosis

Healthy screenee

Proactive patients who turn up to screening opportunities take better care of themselves are less likely to have a positive result

Less likely to smoke, drink too much, have low income

More likely to exercise, eat healthily, attend healthcare at other times

Internal locus of control

Length time

Screening appears to improve prognosis because slow-forming conditions are detected and treated earlier than they would compared to waiting for symptoms to start

e.g. 500 slow forming and 500 fast forming cancers happen each year

Slow forming – no symptoms and better prognosis

Fast forming – obvious symptoms and poor prognosis

Screening can detect lots of slow forming, but not many fast cancers

Because slow has better prognosis, it appears that screening helps outcome, but actually just selects a high proportion of slow cancers

Lead time

A screening test diagnoses something earlier but has no impact on outcome

Appears to increase survival time, but doesn’t

Screening detects a disease

Symptoms start

Death

Screened patients

Non-screened patients

Lead time

Overdiagnosis

Patients are diagnosed with a condition which isn’t going to affect their life expectance

e.g. prostate cancer diagnosis in old men

Get a PSA blood test done, high result but managing with symptoms ok

Now told they have cancer – anxiety, health insurance etc.

A new blood test is developed for rheumatoid arthritis. What is the sensitivity, specificity, PPV and NPV?

Diseased Non-Diseased

New test positive 250 26

New test negative 3 150

Sensitivity = 250 / (250+3) = 98.8 %

Specificity = 150 / (150+26) = 85.2 %

PPV = 250 / (250+26) = 90.5 %

NPV =150 / (150 + 3) = 98 %