Recursive calls using API-Easy JS

I'm trying to setup API test using API-easy and I haven't been able to test the case where I need to use the parameters recovered from one call and send them in another call. Any one has an example of this? I know this should be very straight forward, I just haven't been able to figure it out.

var APIeasy = require('api-easy'),
    assert  = require('assert');

var _id_user;

var suite = APIeasy.describe('Test User');
suite.use('localhost', 3000)
     .discuss('Test')
     .setHeader('Content-Type', 'application/x-www-form-urlencoded')
      .post('/user/authenticate', {data: '{"email":"emailuser@email.com","password":"123456"}')
        .expect('should respond with ID user', function (err, res, body) {
            _id_user = body;    //  I need this result to be sent in the next call .post()
        }).next()
      .post('/user/validate',{ data : _id_user}) // this result always comes null 
        .expect('should respond TRUE', function (_err, _res, _body) {
 }).export(module);

how do I get this result in _id_user and send the call to .post() thanks