CD34 and Alzheimer disease: Possible reasons for the heterogeneity across the studies can be attributed to clinical differences in methodology such as variations in the diagnostic criteria used for enrolling patients with AD, the demographics of the participants who are diverse in age, gender, ethnicity, stage of AD, as well as comorbidities, and publication bias such as selective reporting which may lead to a skewed perception of the diagnostic efficacy of CD34+ as a biomarker [12].