How do I call a SQL Server stored procedure from VB6 using ADO without specifying all parameters?

I am calling a SQL Server Stored Procedure from Visual Basic 6, using ADO.  The current (and working) code is as follows:
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' Set up a command object for the stored procedure.
With ADOcmd
    .ActiveConnection = conProdDB
    .CommandText = "pxPatientInsert"
    .CommandType = adCmdStoredProc

' Set up a return parameter.
    .Parameters.Append .CreateParameter("@ReturnValue", adInteger, adParamReturnValue)
           
' Set up input parameters.  We have found that Must have all parameters listed in order even if we don't have an input for it.
    .Parameters.Append .CreateParameter("@ParticipationId", adInteger, adParamInput, , 40027)
    .Parameters.Append .CreateParameter("@LastName", adVarChar, adParamInput, 50, Null)
    .Parameters.Append .CreateParameter("@FirstName", adVarChar, adParamInput, 50, Null)
    .Parameters.Append .CreateParameter("@ClientSiteIdentifier", adVarChar, adParamInput, 25, Null)
    .Parameters.Append .CreateParameter("@LabPatientCode", adVarChar, adParamInput, 50, "9")
    .Parameters.Append .CreateParameter("@LabPatientCode2", adVarChar, adParamInput, 50, "Next test")
    .Parameters.Append .CreateParameter("@BirthDate", adDate, adParamInput, , "10/7/1953")
    .Parameters.Append .CreateParameter("@Gender", adVarChar, adParamInput, 1, "M")
    .Parameters.Append .CreateParameter("@Age", adInteger, adParamInput, , 53)
    .Parameters.Append .CreateParameter("@AgeMonths", adInteger, adParamInput, , Null)
    .Parameters.Append .CreateParameter("@IHMASerialNumber", adInteger, adParamInput, , Null)
    .Parameters.Append .CreateParameter("@BatchIdentifier", adVarChar, adParamInput, 50, "9999src")
    .Parameters.Append .CreateParameter("@DiagnosisId", adInteger, adParamInput, , Null)
    .Parameters.Append .CreateParameter("@ClientDiagnosisCode", adVarChar, adParamInput, 75, Null)

' Execute command
    .Execute
    PatID = .Parameters("@ReturnValue")
End With
   
Set ADOcmd = Nothing
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The Stored Procedure code is as follows (just to be complete):
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CREATE PROCEDURE [dbo].[pxPatientInsert]
(
      
      @ParticipationId int ,
      @LastName varchar(50)= null,
      @FirstName varchar(50)= null,
      @ClientSiteIdentifier varchar(25)= null,
      @LabPatientCode varchar(50),
      @LabPatientCode2 varchar(50)= null,
      @BirthDate datetime = null,
      @Gender char(1)= null,
      @Age int = 0,
      @AgeMonths int  = 0,
      @IHMASerialNumber int = null,
      @BatchIdentifier varchar(50) = null,
      @DiagnosisId int = null,
      @ClientDiagnosisCode varchar(75)= null
)
AS
      

INSERT INTO dbo.tbl_Patient
      (ParticipationId, LastName, FirstName, ClientSiteIdentifier, LabPatientCode, LabPatientCode2, BirthDate, Gender, Age, AgeMonths, IHMASerialNumber, BatchIdentifier, DiagnosisId, ClientDiagnosisCode)
VALUES ( @ParticipationId, @LastName, @FirstName, @ClientSiteIdentifier, @LabPatientCode, @LabPatientCode2, @BirthDate, @Gender, @Age, @AgeMonths,  @IHMASerialNumber, @BatchIdentifier, @DiagnosisId, @ClientDiagnosisCode);

Return SCOPE_IDENTITY()
GO
******************************************************************************************************************************

My question:  I would like to call the procedure and send just the parameters that I am interested in, instead of all.  In Query Analyzer, I would do this by:

EXEC pxPatientInsert @ParticipationId=12345, @LabPatientCode='XYZ123', @BatchIdentifier='Junk'

If I try just to include the parameters I need with the current code (appending only 3 input parameters),  the fields get wrong information stored (i.e. LastName gets a value of 'XYZ123' and FirstName gets 'Junk').  How is this done?
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