B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 178
Communication Breakdown; Confusion
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Procedure
Descending on the ANTHM STAR under Washington Center Control; we were cleared to descend 'via the ANTHM' arrival.  We had the ATIS; but because Washington does not assign the runway transition; we set 11;000 in the MCP; which is the lowest we can go without a runway assignment.  When we were switched over to Potomac Approach; reporting altitude passing; descending via to 11;000; and the ATIS; the response we got was '33 Left'.  No 'continue descent via 33 Left transition'; or 'Maintain 11;000 ; expect 33 Left'; or 'continue descent to X thousand; expect 33 Left'(or some variation thereof).  After a few moments; I queried the Controller if we were cleared to continue descent via the 33 Left transition; at which point he responded; 'Continue via the 33 Left transition'.  We continued without further incident. To me this exchange was confusing and could lead to a misinterpretation of the approach clearance. This is the second time this has happened to me and on advice from the Chiefs I am filing this report because this confusion needs to be addressed.  Standardized phraseology needs to be used.  I think the Controllers are assuming that we will fly the transition because the ATIS has the active runway information.  But I don't think this is correct.  The Controllers need to be specific.
B737-800 Captain experiences confusion during the ANTHM RNAV to BWI due to the late runway transition assignment by PCT Approach; and their reluctance to issue the transition on the initial check in.