Question

how do physicians determine the dose of antibiotic to administer?

Answer 1

The doctors or clinicians chose the antibiotic based on the susceptibility of the causative organism for the drug. Antibiotics are only chosen if the physician suspects a bacterial infection. Viral infections are subscribed antibiotics as they are ineffective. The diagnosis is bases on symptoms, age of patient, medical history and co-morbidities. Blood tests also determine if the infection is bacterial or viral. If the causative organism is known, then least toxic, narrow spectrum bactericidal and cost effective antibiotic is selected for use. Clinical data is also consulted to prescribe the most effective antibiotic.

In order to select the appropriate dose to be administered, the doctor should know if the drug is narrow spectrum or broad spectrum or whether the causative bacteria are gram positive or gram negative. Microbial cultures and gram staining can identify the Gram staining status of the bacteria. Hence, the doctor should send samples for microbial culture and biochemical test before starting the antibiotic therapy. The bacteria can also be cultured and tested for antibiotic resistance. Agar diffusion method (Kirby Bauer) is the best method to determine which antibiotics the causative bacteria is resistant or sensitive to. The most sensitive (inhibition of growth) is selected for. It should be determined whether the drug is bactericidal or bacteriostatic. Minimum inhibitory concentration of the antibiotic should be determined so that the antibiotic that inhibits growth in minimal time should be considered for therapy. If the bacteria are drug resistant, then novel antibiotics should be tested against the bacteria. Also drug should have minimal collateral damage, especially in case of antibiotic resistance.

Another factor that determines the dose of antibiotic used is the route of administration. If the antibiotic is administered intravenously, then lower doses are required. In case of oral administration, then higher doses are required, as the drug will be metabolized and then absorbed in blood stream. In case of oral administration, multiple doses may be required to obtain effective concentration of the drug in blood. Similarly, the dose depends of which organ the infection has affected. There may be several inhibitions to drug absorbance in these tissues which must be considered. Doctors can consult clinical data to understand the most effective route of administration, the peak urine or plasma concentration obtained while determining the dosage. The doctor should also understand whether any serum proteins may inhibit availability of the drug to target organs. The drug should also not cause drastic side effects to the patients such as allergic reactions, digestive problems etc. The dose should be adjusted based on sensitivity issues of side effects in the patient.