Written By: Dr Anitha Anchan | Updated : May 9, 2016 5:18 PM IST
Infectious diseases can be colds and coughs, septicemia, STDs, etc. They are caused by bacteria, viruses, fungi or parasites. Many harmless and useful microorganisms are present in your body. But under certain conditions, they may cause infections. You may catch some infectious organisms from another infected person or an insect bite. Ingesting contaminated food or water may also cause infectious diseases. Your blood sample can be used to identify microorganisms that cause your infections.
What are the blood tests for infectious diseases?
Your blood sample may be used to diagnose an infection and identify the causative organism. One or more of the following tests may be done:
1. Complete blood count
Tests like complete blood count (CBC) or blood chemistry panel may help indicate a potential infection or identify its source.
2. Blood culture
A blood culture can give evidence of bacterial, fungal and viral growth. In this blood test, your blood sample drawn is placed in a solid or gas substance containing nutrient (culture media) and incubated in a controlled environment. The sample is then observed for signs of microbial growth like the color change, cloudiness, bubble formation, the formation of distinct colonies, etc. To detect a virus in the blood, the sample is injected onto a culture media with cells in which the virus can grow. Here's how to read your medical test report: Blood glucose tests
3. Microscopic examination of blood sample
The infectious bacteria may sometimes be identified by staining with red and purple stains and examining it under a microscope. The distinctive shape, size and stained color help determine the type of bacteria. Changes in the cells of a viral culture media observed under a microscope may indicate viral growth.
4. Antibiotic susceptibility test
When an infectious agent has been identified, an antibiotic susceptibility test may be done to determine what antibiotics are effective against them so that the most appropriate treatment can be started sooner.
5. Antibody test
The antibody tests check for antibodies that the body produces in response to an infection.
6. Polymerase chain reaction test
Polymerase chain reaction (PCR) test finds either the RNA or the DNA of microbe in a small sample of blood. Fungi can be rapidly and accurately detected by PCR-based molecular approaches.
Reading your blood test report
1. WBC count in the complete blood count test measures the number of white blood cells. Normal range is 4,500-10,000 cells/microliter (mcL). Increased WBCs may be a sign of infection.
2. The bacteria or fungi grow to sufficient detectable numbers in a culture media in a couple of days or longer. Viruses may take several weeks. Usually, 2 to 3 blood cultures are done before the result is considered negative or positive:
Negative blood culture (no growth) - no disease-producing agents are discovered. The probability of you having a blood infection caused by bacteria and fungi is low. Failure to recover the infectious agent present in the blood is termed false negative . It can occur due to little blood sample collection, poor handling of the sample, slow-growing bacteria unfavorable incubation environment, etc. Here's how to read your medical test report: Lipid profile.
Positive blood culture - disease-producing agent discovered in the blood sample. You are likely to be infected with the pathogen. Further lab tests can help identify the specific bacteria or fungi causing the blood infection. A false positive result, i.e., a positive blood culture even in the absence of pathogens in the blood may occur due to contamination of the blood sample most likely with the skin bacteria. Here's how to read your medical test report: Blood culture test.
3. If bacteria retain the purple gram stain and look blue under the microscope, they are classified as gram-positive. If they do not retain the stain, they look red under the microscope and are classified as gram-negative.
4. The antibiotic sensitivity test shows how sensitive the microorganism is to a particular drug.
5. Detecting the presence of an antibody, i.e., a positive result in an antibody test may mean you are infected. But a positive finding may not always necessarily mean that you are still infected as antibodies remain in the blood long after your infection has been cured. False-negative results can occur if the test is conducted in the early stages of infection when antibodies are not yet produced.
6. PCR test result is reported as positive or negative. A positive report means that the disease-causing agent has been detected in your blood whereas a negative result means that no pathogen is detected.
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