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What blood tests reveal about infections

What blood tests reveal about infections

When an infection is suspected, a blood test is often used to guide the diagnosis and subsequent course of treatment

A blood investigation is done for a variety of reasons and each test has a specific interpretation

Blood tests indicate a wide range of conditions. From detecting the presence of a pathogen and evaluating the clotting mechanism to analysing the body’s immune response, blood investigation is the primary test done in case of a suspected infection with fever.

A routine blood test is a standard test which is not age or gender specific. It establishes a baseline for one’s health status.

Blood tests to guide diagnosis

Dr Anup R Warrior, internal medicine specialist, Aster Medcity, Kochi, explains the three broader investigations done through blood tests in case of a suspected infection.

  • Test for inflammatory markers: Inflammatory markers are non-specific blood tests used to detect inflammation which could be caused by many conditions like rheumatological ailments or malignancies. These tests must be seen in the backdrop of a suspected infection. They indicate whether it is a viral infection or a bacterial infection, which requires antibiotics. The higher the inflammatory biomarker, the more severe the infection. Erythrocyte sedimentation rate (ESR), C reactive protein (CRP) and procalcitonin are the three common inflammatory markers.
  • Microbiological tests: These check the kind of bacteria, fungi and viruses that have resulted in the infection through RTPCR testing (widely used in Covid-19, dengue detection). This also includes antibody tests or serological tests which could indicate indirect evidence of infection; it measures an individual’s response against an infection and not exactly the presence of a bug.
  • Host response markers: Not available widely, they are largely under research. The specific host response markers will go up only when there is an infection.

Dr Sudha Menon, internal medicine specialist, Fortis Hospital, Bengaluru, says a blood investigation is done for a variety of reasons and each test has a specific interpretation. “It can vary from diagnosing anemia to kidney or liver problems, cholesterol issues and blood culture tests to pick up infections. A standard blood test done for a person suspected of an infection is the complete blood Count or CBC,” says Dr Menon. The CBC assesses three different aspects — the white blood cell count, hemoglobin and platelet count.

Complete blood count (CBC) test

This test can indicate whether the person has a viral or bacterial infection and if the infection is severe. It gives a fair idea of whether the person needs to be hospitalised. CBC assesses three different things – the white blood cell count, hemoglobin and platelet count.

Dr Sudha explains what’s typically seen in a CBC report and how it directs the treatment:

  • White blood cell (WBC) count

WBCs fight against infections. Normal WBC count is 4,000-11,000 per microliter of blood.

A high WBC (above 11,000) is typically a sign of bacterial infection. Low WBC suggests airborne infections. WBC count can be low, normal or high. While the WBC count goes up in case of pneumonia and urinary infections, it remains normal in case of typhoid and goes down during dengue.

  • Hemoglobin

Hemoglobin is the protein in the red blood cells which carries oxygen. The normal range is 12-15 g/dl.

If it is less than ten, it indicates malaria. Low hemoglobin also indicates anemia.

  • Platelet count

Platelets prevent bleeding. The normal platelet range is 1.5 to 4.5 lakh per microliter of blood.  

A rapid decline in platelet signals dengue.

According to Dr Menon, in some of the infections, the parameters can vary. “For example, malaria can cause anemia, low platelet count and fever. In dengue, the infected person will have a low blood count. The platelets which are reasonably fine in the first two to three days of the fever further drop dramatically when the fever settles down. A combination of low WBC and low platelet count (less than 1.5 lakhs) is a sign of dengue or malaria,” she says.

Dr Menon adds that a low WBC count with a normal platelet count suggests an airborne viral infection like covid or influenza H1N1. “The interpretations depend on the clinical scenario. If a person has a fever and a normal WBC count, then there is no need to prescribe antibiotics immediately and we could wait and watch,” says Dr Menon.

D-Dimer-inflammatory marker

During the pandemic, non-specific inflammatory markers such as CRP, D-Diemer and the ferritin test (ferritin is a blood protein containing iron. The low level of ferritin shows iron deficiency) were closely associated with complications of covid, points out Dr Warrior. D-Diemer is a marker of dissolving clots. “It means there was a blood clot, which is now dissolving. It can go up in complications following an infection, as we had seen in the case of covid,” he says.

The timing of the blood tests matters

According to Dr Warrior, the time at which the tests are conducted matters. “It [the infection] takes a certain number of days to manifest. C reactive protein starts to go up by day two or three. If the test is done on the first day of fever, it may be low and hence a false negative,” he says.

Besides, up-trending (spiking) CRP indicates steroid usage and hence medicines taken by the person must be checked, says Dr Menon.

Dr Warrior adds that certain markers take a longer time to return to the normal range. For example, the ESR tends to go up slowly and come down slowly. “Even if a person had an infection and is cured, the ESR will take weeks to come to the normal range. Having a high ESR doesn’t mean there is still an infection. Hence it should not be a matter of worry,” says Dr Warrior.


  • Fever can be a symptom of an infection. Blood investigation gives an idea of what pathogen could have led to the infection.
  • Complete blood count or CBC is a standard blood test that gives a count of the white blood cells, hemoglobin levels and platelets.
  • There are also inflammatory markers which hint at underlying infections.

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