C# - VS2010 - Capturing Invisible GridView Column Value

How do I pass a GridView column value to the code behind with out making it visible? In the following code, I would like to capture the ContactID with out making it visible in the GridView.

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
DataSourceID="SqlDataSource1"
OnRowCommand="_OnRowCommand"
>
<Columns>
<asp:BoundField DataField="ContactID" HeaderText="ContactID"
SortExpression="ContactID" InsertVisible="False" ReadOnly="True" />
.....other columns in the grid....
<asp:ButtonField ButtonType=Button CommandName="Remove" Text="Remove" />
</Columns>
</asp:GridView>

I would like to capture ContactID as follows. I can not do so with out making it visible = true in the GridView.

if (e.CommandName.Equals("Remove"))
{
int index = Convert.ToInt32(e.CommandArgument);
int ContactID = Convert.ToInt32(GridView1.Rows[index].Cells[0].Text.Trim());
}