Tst >15 mm, quantiferon negative. Is it latent tb? Would you treat it?

Chest X-ray &treat. A positive tuberculin test more than 15 mm needs to be followed with a chest x-ray.If the x-ray is neg you need to take prophylaxis with inh for six months.If the x-ray is positive you will need a sputum culture for acid fast bacilli and will need to be treated with multiple drugs for full course of therapy and will need repeat sputum test to ensure you are not able to infect others.
See below. Sometimes TB skin tests are placed wrong--quantiferon is supposed to be more accurate than TB skin test. Your doctor can have an id doctor comment on case to give you peace of mind.