Sources of bias in specimens for research about molecular markers for cancer

Abstract

Claims about the diagnostic or prognostic accuracy of markers often prove disappointing when "discrimination" found between cancers versus normals is due to bias, a systematic difference between compared groups. This article describes a framework to help simplify and organize current problems in marker research by focusing on the role of specimens as a source of bias in observational research and using that focus to address problems and improve reliability. The central idea is that the "fundamental comparison" in research about markers (ie, the comparison done to assess whether a marker discriminates) involves two distinct processes that are "connected" by specimens. If subject selection (first process) creates baseline inequality between groups being compared, then laboratory analysis of specimens (second process) may erroneously find positive results. Although both processes are important, subject selection more fundamentally influences the quality of marker research, because it can hardwire bias into all comparisons in a way that cannot be corrected by any refinement in laboratory analysis. An appreciation of the separateness of these two processes-and placing investigators with appropriate expertise in charge of each-may increase the reliability of research about cancer biomarkers.

Fig 1.

The fundamental comparison in experimental and observational study design. (A) Randomized controlled trial (RCT) of therapy (experimental design). The comparison in an RCT of treatment begins with random allocation of subjects (eg, humans or animals) to the treatment or comparison group, so that baseline equality is assured in the compared groups. The entire fundamental comparison is done under the observation or supervision of the laboratory investigator, making potential sources of bias easier to anticipate and control. Although selection of subjects affects generalizability—“to whom” results may apply—the selection of subjects is not involved in the comparison itself. Specimens (eg, tissue or blood) may be taken from subjects at different times (eg, at the end of the study to assess an outcome of treatment [as shown in the figure]), or just after random allocation to assess prognosis or prediction. (B) Observational study of diagnosis. The comparison begins with subject selection that is not randomized; the process of subject selection may, itself, introduce bias, or systematic differences between the compared groups. This kind of bias, occurring before specimens ever reach the laboratory, may be totally unknown to the laboratory investigator and may fatally flaw any comparison done in the investigator's laboratory. R, random allocation.