I want to do correct DX coding for Child Nurologist.
Most of the time claims denied saying that Primary DX code is incorrect.
Claim was submitted to BCBS ,
CPT 99214
DX : 1) 314.01(attention deficit disorder,without mention of hyperactivity)
2)299.00 (Autism)
3)318.00 (mental retardaion,moderate)
Clm rejected from clearing house with error massage 'SUPPLEMENTAL DIAGNOSIS CODE IS MISSING for 318.00 '

There are lots of other examples.
From where do I know which is the primary and which is the secondary Dx?
Thank You,