There is increasing interest in leveraging electronic health data across disparate sources for a variety of uses. A fallacy often held by data consumers is that clinical data quality is homogeneous across sources. We examined one attribute of data quality, completeness, in the context of electronic laboratory reporting of notifiable disease information. We evaluated 7.5 million laboratory reports from clinical information systems for their completeness with respect to data needed for public health reporting processes. We also examined the impact of health information exchange (HIE) enhancement methods that attempt to improve completeness. The laboratory data were heterogeneous in their completeness. Fields identifying the patient and test results were usually complete. Fields containing patient demographics, patient contact information, and provider contact information were suboptimal. Data processed by the HIE were often more complete, suggesting that HIEs can support improvements to existing public health reporting processes.