a) 466.0, 493.92 or b) 493.90?
Wisconsin Subscriber Answer: Neither option "a" nor "b" is a good choice. Although you should list 466.0 (Acute bronchitis) as the primary - and possibly only - diagnosis, you should probably avoid using an unspecified asthma code (493.9x, 493.x0). When you don't have access to the FP's notes, go back to the physician for more information, if the documentation supports assigning an asthma code at all.
If the FP simply mentions that the patient has a history of asthma, you may want to reconsider reporting 493.xx. Part of the decision depends on the service's place of service:
Inpatient: You will probably want to code the secondary diagnosis because you need as many comorbidity ICD-9 codes as possible to support the assigned E/M level in a facility setting.
Outpatient: Look at whether the visit addresses the asthma. If the patient's asthma is not causing any problems, the condition may not warrant using an ICD-9 code. But if the condition affects the encounter - for instance, the patient's asthma medication impacts the bronchitis' treatment - report the condition.
Be careful: Most insurers will deny unspecified asthma code 493.9x. You may have to submit additional notes and ultimately have to select a specific code.
Better method: Before filing the claim, get the information you need to avoid using an unspecified code. You don't indicate that the patient's asthma is exacerbated (493.x2, ... with [acute] exacerbation) or that the condition requires treatment with a nebulizer, so the patient's asthma is probably stable. In this case, you would use a fifth-digit subclassification of 1 (... with status asthmaticus).
You can assign the appropriate specific asthma code with one more piece of information: the patient's asthma type. Use 493.01 for a stable extrinsic (typical) asthmatic and 493.11 for a stable intrinsic (less common) asthmatic.