Coding hospital labs

Why are the rules for assigining ICD9 codes to outpatient hospital labs so different from coding lab tests done in a physician's office? I am a CPC, CPMA, who is learning some hospital coding by coding outpatient lab tests from physicians requisitions, and what I thought I knew doesn't always apply. Very frustrating!

Hospital Lab Coding

What are the issues that you have encountered? Rules that govern hospital billing & coding do vary quite a bit different from physician rules in certain areas. Although there are similarities between facility outpatient & physician reporting, there are other factors that hospitals may be required to consider.