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Present but Null Fields

One question that arises in HL7 quite often is how to tell if a field is present but null in an HL7 message. When a field is absent from a message it should mean that the system does not use that field, or that the field has not changed. However, what if a system does support the given field, but the value of the field is null? The HL7 protocol requires a method of making this clear.

To make present but null fields clear, you must put two double-quote characters together in a field like this:

OBR|||||""||||

This is a very common technique used in many real-world implementations.