Here's an article  why you should avoid long term ingestion of antibiotics.
Independent lab test results
At DQ, we commission a reputable independent lab to randomly pick chickens from outlets selling our chickens to be tested.  The lab will go to outlets selling our chickens, pick the same chickens that consumers are buying and then test them.  They do this without telling the outlets or informing us prior to their going to the outlets. They test for common antibiotics used in the industry.  In addition they test for beta argonist and for nitrofluran. 

We arrange for this to act as an independent assurance for our customers on the quality of our chickens.
Here's a copy of the random lab test results of our chickens.
Click here to read.
Antibiotics and Poultry
Modern Intensive Farming and Antibiotics
Modern farming is economically efficient.  A modern closed chicken house provides about 0.75 sq ft of space per chicken.  Open houses may provide up to 1.25 sq ft per chicken while modern intensive free-range farms in Asia may provide up to 2 to 3 sq ft of space per chicken.

Because of the dense population,  the farmer is always worried about disease breaking out.  One way to prevent disease from breaking out is to give the chickens sub-therapeutic antibiotics.  This is antibiotic given every day as a preventive, not to cure any specific disease.  Antibiotics also reduce the microbial load in the guts of chickens, resulting in better absorption of nutrients and hence, faster growth.  In this instance, antibiotics are given as growth enhancers.

Though it is possible that there are intensive farms that do not use sub-therapeutic antibiotics, in our view, it is highly unlikely.  There is too much risk involved for the farmer.
Antibiotics Test
Some farmers say that their chickens are antibiotics residue free or antibiotics free.  If their chickens are from intensive farms, then we need to know the sensitivity of their tests.  Tests can be sensitive to parts per thousand, parts per million or parts per billion.  Obviously tests done to parts per thousand will still be detectable if tested for parts per million.  Generally antibiotics free animal products should be tested to parts per billion.
In addition, these tests should be done by an independent lab, not inhouse labs.  Tests should include tests for nitrofluran.  Nitrofluran is banned and have insidiously entered into our food chain.