>>>But I get a syntax error:
>>>
>>>Msg 207, Level 16, State 1, Line 9
>>>Invalid column name 'statusfk'.
>>>
>>>How can I refer to the statusfk field?
>>
>>You have one extra )
>
>Where? I see two opening and two closing.
>
>FWIW, I got this to work:
>
>

select 1 
>	from ClaimItems inner join Claims on cit_clmfk = clm_pk
>	where dbo.claimitemstatus(clm_forwarded
>				,cit_received
>				,(select com_overduedays
>				from companies
>					inner join Clients on cli_comfk = com_pk
>					inner join Claims subclaims on clm_clifk = cli_pk
>				where subclaims.clm_pk = claims.clm_pk)) = 3

Change this to:

select 1 
	from ClaimItems inner join Claims on cit_clmfk = clm_pk
	where dbo.claimitemstatus(clm_forwarded
				,cit_received
				,(select SUM(com_overduedays)
				from companies
					inner join Clients on cli_comfk = com_pk
					inner join Claims subclaims on clm_clifk = cli_pk
				where subclaims.clm_pk = claims.clm_pk)) = 3

This will help avoiding subquery returned more than 1 record error in a future.

I still think using SCALAR function this way is not good for performance.