Wiki which diagnosis is the principal diagnosis?


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I am studying for my ccs exam and am confused about the following coding scenario.

Which is the primary diagnosis in "Degenerative joint disease secondary to Hyperparathyroidism"?

The answer is these type of instances when you have something secondary to something or something due to something, how do you know which one to code first?

Please help!!! (is there any reference material I can read up on this?)

Amber, CPC
principal diagnosis?

I would start with the coding guidelines in the front of the ICD-9 book. There are several sections that list sequencing of codes, secondary codes, and associated illnesses. Try this first and see if your question can be answered there.
Which was the reason for the admission (after study)? That is the definition of principal dx.