While several standards for metadata describing clinical studies exist, comprehensive metadata to support traceability of data from clinical studies has not been articulated. We examine uses of metadata in clinical studies. We examine and enumerate seven sources of data value-level metadata in clinical studies inclusive of research designs across the spectrum of the National Institutes of Health definition of clinical research. The sources of metadata inform categorization in terms of metadata describing the origin of a data value, the definition of a data value, and operations to which the data value was subjected. The latter is further categorized into information about changes to a data value, movement of a data value, retrieval of a data value, and data quality checks, constraints or assessments to which the data value was subjected. The implications of tracking and managing data value-level metadata are explored.