If this was ordered as screening, then screening must be your primary diagnosis. You may add 733.90 as a secondary diagnosis, but you cannot make it your primary diagnosis.
Look at the Official Guidelines for ICD-9-CM:
"A screening code may be a first listed code if the reason for the visit is specifically the screening exam. It may also be used as an additional code if the screening is done during an office visit for other health problems. A screening code is not necessary if the screening is inherent to a routine examination, such as a pap smear done during a routine pelvic examination.
Should a condition be discovered during the screening then the code for the condition may be assigned as an additional diagnosis."
And from CMS in the Claims Processing Manual, Chapter 23
"10.1.5 - Diagnostic Tests Ordered in the Absence of Signs and/or
Symptoms
(Rev. 1, 10-01-03)
When a diagnostic test is ordered in the absence of signs/symptoms or other evidence of
illness or injury, the testing facility or the physician interpreting the diagnostic test should
report the screening code as the primary
diagnosis code. Any condition discovered during
the screening should be reported as a secondary diagnoses."