Try this
protected void ddlAddLabTest_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList ddlLabTest = (DropDownList)sender;
DataGridItem row = (DataGridItem) ddlLabTest.NamingContainer;
DropDownList ddlAddLabTestShortName = (DropDownList)row.FindControl("ddlAddShortname");
ddlAddLabTestShortName.SelectedIndex = intSelectedIndex;
}