RadTextBox textchanged event

[Neetha]

RadTextbox textchanged event is firing before i complete typing the text in the textbox.
So I am trying onBlur client event, but i need the functionlity in server side. How do i accomplish it ?

 <telerik:RadTextBox ID="txtPurchaseOrder" AutoPostBack="true" runat="server" EmptyMessage="" MaxLength="30" style="text-transform: uppercase;" >
              <ClientEvents OnKeyPress="OnKeyPress" OnBlur="Blur" />      
              </telerik:RadTextBox>

<script> function Blur(sender, eventArgs) {
            // need to call POChange method, pagemethods.POChange not working
        }
</script>

code-behind :

 protected void POChange()
        {
            if (txtPurchaseOrder.Text != "")
            {
                string strSoOrder = BookingData.CheckPONumer(txtPurchaseOrder.Text);
                if (strSoOrder != null || strSoOrder != "")
                {
                    txtPurchaseOrder.Attributes.Add("onblur", "javascript:return confirm('The Purchase Order used is the same used on previous SO# do you want to continue?');");
                }
            }
        }

How to get this working?