Here is my code:
$service = new NetSuiteService();
$getRequest = new GetRequest();
$getRequest->baseRef = new RecordRef();
$getRequest->baseRef->internalId = "custitem3";
$getRequest->baseRef->type = "itemCustomField";
$result = $service->get($getRequest);
echo "<pre>";
var_dump($result);
echo "</pre>";
And the result is: That record does not exist.
But that custom item field show in Set up > Customization > Item fields in netsuite backend