Question

3. What is LEAD TIME BIAS? How does LEAD TIME BIAS affect the statistics showing survival rates for cancer? How does this affect YOUR feeling towards screening tests?

Answer 1

Answer with discussion and explanation:

There are usually no symptoms at the onset of most of the cancers/diseases. As the cancer or disease progresses and damages the normal physiology of body, it begins to produce symptoms. After appearence of symptoms or signs person goes to doctor and his condition is diagnosed.This is the usual scenario, where a person having a disease condition seeks medical attention after becoming symptomatic.

Now consider we have a effective screening test which can detect cancer in asymptomatic period. Thus will lead to early diagnosis of the cancer before development of any sign or symptom.This extra time gained between early detection to onset of first symptom is termed as lead bias or lead time.

This lead bias many times give false impression of increased survival. Let us consider a hypothetical example: Mr X has onset of lung cancer in October 2019, but he developed symptom (coughing out of blood) after 2 months in December 2019 and then approached a doctor to be diagnosed as a case of lung cancer. Sadly, he died in April, 2020.Hence he survived for 4 months after diagnosis and 6 months from the actual onset of disease.

Now, suppose a screening test detected his lung cancer in November 2019 (1 month before being symptomatic).

This 1 month period will be lead time due to early diagnosis of his condition. It will appear that he survived 5 months after the diagnosis by screening test whereas he survived only 4 months after symptomatic diagnosis. But his actual survival from the onset of disease is 6 months in both scenarios. Hence, screening test lead time many times give false impression of increased survival.

But considering pap smear as screening test in case of cervical carcinoma, which can lead to early detection of cervical carcinoma cases before adevelopment of any symptom or metastasis, allows for early definitive treatment if diagnosed very early. In this case screening test is definitely beneficial in reducing morbidity and mortality if used properly and early.

So, use of screening test will depend on many factors like the condition for which it is being used (how aggressive the tumor is), sensitivity, specificity, positive predictive value and negative predictive values of the screening test.