A new screening test was devised to detect pancreatic cancer at early stages using a serum marker (CA19-9) of the disease. A study of this new test showed that its use prolongs the survival of patients with pancreatic cancer by several months. The researchers concluded that use of the test improves the prognosis of patients with pancreatic cancer. Which of the following is a potential problem with this conclusion?

Question

A new screening test was devised to detect pancreatic cancer at early stages using a serum marker (CA19-9) of the disease.  A study of this new test showed that its use prolongs the survival of patients with pancreatic cancer by several months.  The researchers concluded that use of the test improves the prognosis of patients with pancreatic cancer.  Which of the following is a potential problem with this conclusion?

Quizplus · Accepted Answer

Lead-time bias should always be considered while evaluating any screening test. This bias occurs when there is an incorrect assumption or conclusion of prolonged apparent survival and better prognosis due to a screening test. What actually happens is that detection of the disease was made at an earlier point in time, but the disease course itself or the prognosis did not change, so the screened patients appeared to live longer from the time of diagnosis to the time of death. (USMLE tip: think of lead-time bias when you see "a new screening test" for poor prognosis diseases like lung or pancreatic cancer.) (Choices A, B and E) Observer's bias, measurement bias and ascertainment bias refer to misclassification of an outcome and/or exposure (e.g., labeling diseased subjects as non-diseased and vice versa) and are related to the design of the study. The scenario described does not mention how the study was designed. (Choice D) Although the results of the study could be potentially confounded, there is no information on how potential confounders were treated during the design or analysis stage of this study. Educational Objective: Understand the concept of lead-time bias in screening tests. The typical example of lead-time bias is prolongation of apparent survival in patients to whom a test is applied, without changing the prognosis of the disease.

A New Screening Test Was Devised to Detect Pancreatic Cancer