How often are sputum cultures false negative? My son is currently recovering from a month of sickness including sinusitis that then settled into his chest. His sputum culture was taken 3 days after he finished 10 days of amox and had just started 3 days of augmentin. His sputum was thick yellow good sample and came back normal. I suppose he could have viral pneumonia but I almost wish it was something so we could do something besides empiric therapy. He is being treated with oral antibiotic for one month now and getting better along with albuterol/pulmicort. Any insights?