Bacterial vs viral & antibiotic duration

AOA Dr. Taqi.
I have a couple of questions which may sound very trivial but they are often asked in the viva.
1. How do you differentiate between a Viral Infection and a Bacterial Infection?
2. What is the optimal duration of antibiotic therapy?
Dr. Hamza

Thank you for the question and walaikumsalaam Dr Hamza,

As usual these questions are looking for a method of response rather than a particular response. There is no absolute right or wrong. I would fashion my answer as follows:

Differentiating a bacterial from a viral infection is a matter of probability rather than certainty. The only true way of differentiating the two would be to run expensive, time consuming tests such as body fluid cultures, PCR for viral DNA. Even with the aid of such tests we would have the additional problem of working out the relevance of any results. Not everything cultured is the cause of an infection, throat swabs and beta hemolytic streptococcus being a good case in point. 

In a clinical setting I would say to the patient, ‘ We can’t tell the difference with 100% surety. But we can make an educated guess based on probability. Most healthy people when having simple infections such as sore throats usually have viral illnesses.  It takes the production of antibodies directed against the virus to destroy the virus. This takes 3 days (reactivation of Plasma cells with a memory) if you have been exposed in the past and it will take about 5-7 days if this is a new virus. It would thus be reasonable to wait for a period of 7 days and let the body do its natural work. If you find the infections gets significantly worse especially around day 3-4 this could imply a secondary bacterial infection in which case I would like you to come back for a review. If you continue to be unwell beyond 7 days I would like to review earlier or at any time where you are significantly worried or concerned by new symptoms.


Optimum duration of antibiotics:

The examiners are looking I would have thought for awareness of a spectrum of answers from one day as in bacterial vaginosis and a single dose metronidazole regime, to three days for uncomplicated UTIs to many months in cases of bacterial endocarditis to continuous for paediatric patients with recurrent UTIs. Other factors that you would take into account are the particular circumstances of the patient (elderly, immunocompromised), social points (children going to school, working people), patient preference and compliance. They may then quiz you on how long you would give an antibiotic before changing it, it may be worth while checking what the latest evidence is on this but from the top of my memory 2 – 3 days should result in some improvement, a lack of any improvement, worsening of condition should prompt a revaluation of the patient and the questions to answer: is the antibiotic  working, is it viral, have I got the diagnosis wrong.

Hope that gives you some ideas on tackling these questions.