AJAX async true returns undefined or null reference error in IE 11

Question

Using this AJAX call with async set to true causes the following function (see below) to fail with undefined error. Clearly it is because the AJAX call is not getting a return value before the browser attempts to use it. If async is set to false this AJAX function works. What am I missing?

function AjaxCmd(cmd) {
var txtRetval;
//alert(cmd);
$.ajax({
    type: "POST",
    url: "/telligent/rpjquery-functions.asp",
    data: "cmd=" + cmd,
    datatype: "html",
    //async Not supported in IE8 or IE9 11082014
    cache: false,
    async: true,
    success: function(data) {
        //alert("success");
        txtRetval = data;
   },
    error: function (request, status, error) {
        //alert(request.responseText);
        txtRetval = false;
    },
    failure: function() {
        //alert("failure");
        txtRetval = false;
    }
});
//return callback( txtRetval ); 
if (txtRetval == "not authenticated") {
    jQT.goBack("#");
    location.reload(true);
    alert(txtRetval);
    return (false);
} else {
    //
}
return (txtRetval);

}

This AJAX call will fail with: unable to get property length of undefined or null reference. This call will work with async set to false:

        var filterstring = AjaxCmd("getfilterstring&intFormID="+intFormID+"&SF="+SF+"&txtFormType="+txtFormType+"&a="+a+"&s="+s);
        var filterstringlength = filterstring.length
        for (var x = 1; x <= filterstringlength; x++)