Why does the javascript onchange event not fire if autocomplete is on?

<input type="text" name="txtTest" value="" onfocus="this.originalvalue=this.value" onblur="if (this.value != this.originalvalue) alert('Test has changed')"/>

'Turn off Auto complete
'(it needs to fire the text change event)
 txtYourTextBox.Attributes.Add("AutoComplete", "off")