Bacterial Culture and Sensitivity Test

Your doctor may order this test if they suspect that you got a bacterial infection. Not all bacteria are equally susceptible to the same antibiotics. Even a single type of bacteria shows variation in sensitivity in different people. This test will help your doctor to determine which antibiotic will work best for you. 

This test has become all the more important in the wake of increasing antibiotic resistance - overuse of an antibiotic will make the bacteria resistant against it. If you already got infected from resistant bacteria, the usual antibiotics won’t work on you. 

The need for this test commonly arises to diagnose conditions like strep throat, pneumonia, urinary tract infection, bronchitis, food poisoning or in case of open wounds. 

Following is a list of some of the culture tests (in which the laboratory technician puts the sample on a growth medium to identify the type of bacteria) your doctor may order, depending on your symptoms:

• Blood culture
• Throat culture
• Urine culture
• Rectal culture
• Stool culture
• Sputum culture
• Wound culture
• Endocervical culture (culture from the walls of the cervix in females)
• Eye culture

The preparation required will depend on the test that your doctor orders for you. No preparation is generally required in case of blood culture, eye culture, rectal culture, stool culture, throat culture, urine culture or wound culture. 

In case of endocervical culture test, women will be asked to avoid tub baths and douching prior to the test. For a sputum culture, you may be asked to have fluids before the test to help thin down the sputum. You may also be asked to brush your teeth before the test to reduce contamination of the sample. 

Inform your doctor if you are on any antibiotics.

Samples for different culture tests are obtained in different ways. For example, in case of throat culture, eye culture, wound culture or endocervical culture, a laboratory technician or nurse will use a sterile swab stick (a small stick with fibre or absorbent material on one end) to take the sample from the appropriate site. The procedure might cause slight discomfort. 

For blood culture, a small amount of blood will be drawn from a vein in your arm using a sterile needle.

For urine and stool cultures, the collection facility will provide you with a sterile cup and explain the process of sample collection. 

For a sputum culture, you may be instructed to take a few deep breaths and then cough to obtain a sputum sample from deep within the chest. 

Some of the tests may require samples more than once. There may be a second round of sampling to confirm the results in the case of blood cultures. For a sputum culture, samples may be taken on three consecutive days. If there is any such requirement, your doctor or nurse will let you know. 

Once the sample is taken, it will be sent to the laboratory for culture. If any growth of harmful bacteria is observed, the technician may need to put the bacteria in special containers with different antibiotics to check which ones can inhibit the growth of the bacteria.

Normal results:

The test results may take some time to be reported, as bacteria can take up to 48 hours to grow. Automated machines, such as the Vitek-2, can perform this procedure much faster and give more reliable results.

Either way, a normal result on this test means that you may not have a bacterial infection. 

However, there have been reports of false-negative results with this test. This may happen due to antibiotic therapy given before sample collection. Antibiotics would delay bacterial growth, which may be seen as negative initially.

Abnormal results:

Abnormal results in this test are reported as positive. An abnormal result will identify the type of bacteria and thus imply the presence of an infection. It will also include a list of antibiotics that would work well against the isolated bacteria. 

Based on the test results, your doctor will start the appropriate antibiotic therapy for you.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.