Key points
- Screening tests people without symptoms; it is not the same as investigating a symptom.
- The proof that screening works is fewer deaths from that cancer in randomised trials.
- Lead-time bias, length-time bias and overdiagnosis make screened cancers look better than they are.
- Every programme has harms: false positives, anxiety and overtreatment.
What screening is
Screening means testing people without symptoms to find a cancer, or a pre-cancer, earlier than it would otherwise be found, when treatment works better. A patient who has a symptom needs investigation, not screening.
The Wilson and Jungner principles (WHO, 1968)
- The condition is an important health problem with a recognisable early or latent stage.
- Its natural history is understood.
- There is an accepted, effective treatment, and facilities to diagnose and treat.
- There is a suitable test that the population will accept.
- There is an agreed policy on whom to treat, the cost is balanced, and screening is a continuing process.
Judging a test
| Measure | Meaning |
|---|---|
| Sensitivity | Of those with the disease, the proportion the test detects |
| Specificity | Of those without the disease, the proportion the test correctly calls negative |
| Positive predictive value | Of those who test positive, the proportion who really have it: falls when the disease is rare |
Three biases every student should know
- Lead-time bias: finding a cancer earlier lengthens the time from diagnosis to death even if death comes at the same time.
- Length-time bias: slow-growing cancers spend longer in the detectable phase, so screening finds more of the kinder tumours.
- Overdiagnosis: finding cancers that would never have caused harm, such as some DCIS and low-grade prostate cancer, leading to overtreatment.
Because of these, survival from diagnosis is a misleading measure. Screening must be shown to reduce deaths from that cancer in randomised trials.
Cancer screening guidelines by age: US and UK
Two of the most quoted systems are the US Preventive Services Task Force (USPSTF) and the UK national programmes (NHS England shown). They differ, which is a useful exam point in itself.
| Cancer | US (USPSTF) | UK (NHS England) |
|---|---|---|
| Breast | Mammography every 2 years, women aged 40 to 74 (2024) | Mammography every 3 years, women aged 50 to 71 |
| Colorectal | Ages 45 to 75: stool test (FIT yearly), or colonoscopy every 10 years, among other options; 76 to 85 selectively (2021) | Home FIT kit every 2 years, ages 50 to 74; colonoscopy if positive |
| Cervical | Ages 21 to 29: cytology every 3 years. Ages 30 to 65: cytology every 3 years, HPV test every 5 years, or both every 5 years (2018) | HPV test first, cytology if positive. Ages 25 to 64; every 5 years if HPV negative (since July 2025) |
| Lung | Yearly low-dose CT, ages 50 to 80, 20 pack-years or more, still smoking or quit within 15 years (2021) | Targeted lung cancer screening with low-dose CT for people aged 55 to 74 who have ever smoked |
| Prostate | Ages 55 to 69: an individual decision about PSA after discussing benefits and harms; not routinely from 70 (2018) | No population programme; informed choice |
Programmes change
Ages and intervals vary between countries and are revised often (the USPSTF cervical and prostate statements are under review). Check your own national programme before quoting a number.
Surveillance is different
People at high risk (Lynch syndrome, FAP, BRCA carriers, cirrhosis, Barrett’s oesophagus) have their own surveillance schedules, starting younger and repeated more often.
Frequently asked questions
At what age does breast cancer screening start?
In the US, the USPSTF recommends mammography every 2 years from 40 to 74. In England, women are invited every 3 years from 50 to 71.
At what age should colorectal cancer screening start?
At 45 in the US (USPSTF, to age 75). In England, home FIT kits are sent every 2 years from 50 to 74.
What is lead-time bias?
Finding a cancer earlier lengthens the time from diagnosis to death even if death happens at the same time, making screening look better than it is.
What is overdiagnosis?
Finding cancers that would never have caused symptoms or death, which leads to treatment that cannot help the patient.
Further reading
- Wilson JMG, Jungner G. Principles and practice of screening for disease. WHO Public Health Papers 34, 1968.
- Independent UK Panel on Breast Cancer Screening. The benefits and harms of breast cancer screening: an independent review. Lancet 2012;380:1778–86.