Lead time bias

Lead time bias refers to the phenomenon where early diagnosis of a disease falsely makes it look like people are surviving longer. This occurs most frequently in the context of screening.

For example, a man with metastatic lung cancer dies at age 70. His cancer was discovered 1 year ago, when he was 69. Therefore, it appears as if he lived for 1 year with the cancer. However, imagine that instead his cancer was discovered on a screening CT scan when he was 65 years old. If he still dies at the age of 70, it now looks like he survived for 5 years with the diagnosis of cancer (the 5 year survival rate is much better), but in fact there was no real change in his survival.

777px-Lead_time_bias.png
From WikiCommons: https://commons.wikimedia.org/wiki/File:Lead_time_bias.svg

This post is part of a series of posts on bias in medical research. You can find the whole bias catalogue here.

You can find more evidence based medicine resources here.

References

Gates TJ. Screening for cancer: evaluating the evidence. American family physician. 2001; 63(3):513-22. [pubmed]

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