Defining a Batch in Continuous Manufacturing

Continuous manufacturing is the way of the future for solid-dosage pharmaceuticals, but one of the challenges for implementation is how to define a batch for quality assurance testing.  There are also other, related questions. When you are running one product continuously, where do you draw the line between out-of-specification start-up or transition material and in-spec product? What can you do about this lost, out-of-spec material? How do you test for quality and maintain data integrity? Other industries that have moved from batch to continuous, such as plastics and food, resolved these issues long ago, and some parts of the pharmaceutical industry have done it too. Some tablet coating is continuous, particularly in cosmetic vitamin coating. Excipients are produced continuously. How can those seeking to move to continuous manufacturing translate what has already been learned to their own application? I’d like to hear from those working in this area. What are the challenges and unresolved questions you face in this task?