Contagiousness of smear negative TB?

Smear negative TB patients are thought to be much less contagious than smear positive patients, but how much so? In this retrospective analysis from the Netherlands using a national DNA fingerprint database, researchers determined 13% of secondary TB cases were attributable to smear negative patients, and that the relative transmission of smear negative, compared to smear positive, patients was 0.24. This study implies that the “no-isolation needed” hospital standard for smear negative TB patients needs to be re-visited, and that the risk of transmission of smear negative patients is less than smear positive, but certainly not insubstantial (abstract)

Danielle Scheurer

Dr. Scheurer is a clinical hospitalist and the Medical Director of Quality and Safety at the Medical University of South Carolina in Charleston, South Carolina, and is Assistant Professor of Medicine. She is a graduate of the University of Tennessee College of Medicine, completed her residency at Duke University, and completed her Masters in Clinical Research at the Medical University of South Carolina. She also serves as the Web Editor and Physician Advisor for the Society of Hospital Medicine.


  1. Jogesh Raja on November 20, 2008 at 9:30 pm

    What is the reliability of sputum collection? Was sputum subjected to MTB PCR DNA testing to see if we missed true sputum negative cases? It is also possible that these patients were sputum negative when tested and became sputum positive later when the TB lesion had breakdown. Does this mean that when we have a patient with proven TB and his/her sputum is negative we should still isolate them and for how long? What happens when we discharge them to community? What advise should we give to protect the community at large and for hoe long he should follow this advise?

  2. Danielle on November 25, 2008 at 2:44 pm

    Great questions posed here. It is a matter of weighing the risk to the patient (of isolation) versus the public health risk of transmission. It is certainly plausible that patients could be intermittantly contagious (such as you suggested); I think the public health community has never posed that a cross-sectional smear-negative patient can never again be contagious. But, this is the first well done study to attempt to quantify what proportion of new TB cases are contracted from “smear negative” patients, and the number is higher than I think anyone expected to see.

Leave a Comment