When you suspect active TB when admitting a patient to the hospital, it turns out it is quite difficult to rule someone in or out. Here is a brief look at how you can get tricked trying to diagnose TB, and some recommendations for workup up these patients.
Skin test and quantiferon gold assay are bad at diagnosing active TB:
- Quant gold cannot exclude active (or latent) TB because 10-35% of patients with active TB will have a negative quant gold assay.
- The skin test will be negative in 50% of patients with disseminated disease and 5-40% of patients with pulmonary disease.
- This online latent test interpreter is excellent: http://www.tstin3d.com/en/calc.html
You really can't RULE OUT active TB with sputum tests. All you can do is have a certain degree of confidence about how many TB organisms are present in the patient's airways. If you have a negative sputum smear, you have detected down to a minimum of 5 thousand colony forming units (CFU)/mL. A negative sputum PCR detects down to 150 CFU/mL. A negative culture detects down to 10 CFU/mL.
Unfortunately, the culture takes weeks, so it's not helpful in the short term when admitting. Many hospitals don't have sputum PCR or Gene Expert testing. That usually leaves us with sputum smear. This will tell you something if the airways are teaming with organisms, but can be falsely negative for disseminated TB or for a lower burden of respiratory disease. The main thing that the negative sputum gives you is confidence that the patient is not contagious to the rest of the hospital.
Imaging is helpful, but also doesn't rule out TB. 5-15% of active pulm TB have negative CXR, CT is better but not absolute.
- If you have even a low suspicion for pulm TB (A respiratory complaint in a patient from an endemic area, HIV, homeless/incarcerated, unexplained wt loss, etc), they need to be on airborne precautions.
- If the sputum smears are negative x3 (you can order them 12 hrs apart), you can be reasonably confident that they're not contagious.
- The skin test and quant gold assays can be negative, but still have pulm or disseminated TB, so they're not very helpful in the inpatient setting. It's worth testing because if positive, they will need TB clinic and likely treatment for at least latent TB and further workup for disseminated disease.
- Your main job in working up active TB is to protect the hospital, patients, their families and staff. An exposure affects hundreds of people, causes lots of hassle, and costs lots of money.