Contagiousness of smear negative TB?

By  |  November 20, 2008 | 

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)


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

    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 November 25, 2008 at 2:44 pm - Reply

    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

About the Author:

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.


Related Posts

By  | October 8, 2015 |  0
by Deepak Asudani, MD, MPH, FHM Whether it is the prompt and expeditious international collaboration to develop an Ebola vaccine, or tardy but promising development of the first anti parasitic malarial vaccine or the fascinating technology utilizing synthetic DNA for vaccine development against MERS, these developments promise to highlight significant strides in vaccine development for […]
By  | August 24, 2015 |  5
  Economists describe preferences in two ways: revealed and stated.  Say, for example, I asked you to implement a penalty program for your team with the goal of decreasing the number of occasions members did not clean their hands after a patient encounter.  Because you know bad hands equal bad outcomes, you’re apt to offer up […]
By  | June 2, 2015 |  1
by Eric Howell, MD, SFHM “Tell me what you know about antibiotics.” That’s the discussion I start with hospitalized patients all the time, right after they ask me to prescribe antibiotics for their simple cough, or other viral-like illness. And, from their perspective, asking for antibiotics makes sense. After all, antibiotics have been the physician’s […]