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Diagnosis and tests

Infection and virus tests

Leukemia and its treatment weaken the immune system. So doctors look for infection twice: before treatment, to find viruses “sleeping” in the body, and during treatment, to quickly find the cause of a fever or other signs. This topic explains which tests are done.

Last updated: ~5 min read
Inside a translucent human silhouette on a dark navy background are several small, quiet, dimmed virus shapes; a soft blue search beam lights them up and a protective shield symbol appears beside each one; no face.
Figure 1.Before treatment, doctors look for viruses “sleeping” in the body.

01Short answer

Before treatment, tests for hepatitis B (HBsAg, anti-HBc), hepatitis C and HIV are usually done; antibodies to cytomegalovirus (CMV), herpes viruses (HSV and the chickenpox–shingles virus VZV) and other infections are sometimes checked too, especially if a transplant is planned. The aim is to know in advance about viruses that could reactivate when immunity is low and to give preventive medicines if needed. During treatment, if there is a fever or other signs of infection, blood cultures (from a vein and from the line), urine, sputum and other samples are taken, a nose and throat swab is tested by PCR for respiratory viruses, CRP and procalcitonin are measured and, if needed, fungal markers and a chest CT are done. When neutrophils are low, antibiotics start straight away without waiting for results; treatment is then refined as results come in.

02Simple explanation

When the immune system is weakened, ordinary germs and viruses that have “slept” in the body for years can become dangerous.

So before treatment, the doctor checks “who is sleeping inside”: hepatitis B, hepatitis C, HIV, CMV, herpes.

If a hidden virus is found, this is not frightening: often a medicine is given to stop it waking up.

If a fever appears during treatment, the doctor looks for the cause: blood, urine, a nose swab, sometimes an X-ray or CT.

Antibiotics start without waiting for results, because time matters a great deal when immunity is weak.

03Scientific explanation

Hepatitis B reactivation. The NHS and MedlinePlus explain that hepatitis B virus can sometimes stay in the liver without symptoms. Chemotherapy and especially immune-suppressing drugs such as anti-CD20 antibodies (for example, rituximab) can reactivate it. So if HBsAg or anti-HBc is positive, the doctor may check the viral DNA and prescribe a preventive antiviral medicine.

Other viruses. Most people have met CMV, HSV and VZV at some point, and the virus stays hidden in the body. With strong immune suppression, especially after a transplant, they can wake up. According to MedlinePlus, CMV is monitored in the blood by PCR; after a transplant this is done regularly. Prevention against herpes viruses is part of many treatment plans.

During a fever. The NCI and ACS stress that with neutropenia the signs of infection can be faint, and sometimes a fever is the only sign. Under IDSA and ESMO guidance, in febrile neutropenia at least two sets of blood cultures are taken (with a central line, from each lumen and from a peripheral vein), and a broad-spectrum antibiotic is started as soon as possible — within the first hour, according to ESMO.

Fungal and respiratory tests. In prolonged neutropenia, fungal markers in the blood (galactomannan, beta-D-glucan) and a chest CT help detect mould infections such as aspergillosis early. Respiratory pathogen panels use PCR on a single nose and throat swab to test for flu, RSV, COVID-19 and other viruses at once.

04What happens in the body?

Before treatment: screening

Tests for hepatitis B, hepatitis C and HIV are done on an ordinary blood sample. CMV, HSV and VZV are often checked as well, and sometimes toxoplasma, syphilis and tuberculosis. Before a transplant the list is longer. The results are confidential and are used only for your care.

How does a blood culture work?

Blood is collected into bottles containing a nutrient liquid and placed in an incubator. If a germ is present, it multiplies and the machine raises a signal. A first answer often comes within 1–2 days, while the name of the germ and which antibiotics work against it can take several days. A negative result is usually finalised after 5 days of monitoring.

Two translucent blood culture bottles stand in an incubator on a dark navy background; small glowing dots multiply in one of them and a calm signal light glows beside it; no people and no needles.
Figure 2.Blood culture: if a germ is present, it grows in the bottle.

Nose and throat swab and PCR

A sample is taken from the nose or throat with a swab; it can be uncomfortable for a few seconds. A PCR panel shows many viruses at once within a few hours. This guides both treatment and isolation measures on the ward.

A translucent sample tube with a swab on a dark navy background; beams of light run from it to several virus symbols of different shapes, and small check marks light up beside them.
Figure 3.One swab, a PCR test for several viruses.

CRP and procalcitonin

These blood markers reflect inflammation. On their own they do not prove or rule out infection, but they help the doctor follow the trend and judge whether an antibiotic is working.

Other samples

Depending on the signs, urine, sputum, stool (for example, for Clostridioides difficile), skin swabs or swabs from the line site may be taken. Sometimes a bronchoscopy or a lumbar puncture is needed. You can ask why each test is being done.

05What happens when a fever starts?

Quick assessment, quick treatment, then fine-tuning.

Report it — Take your temperature and call the team straight away.1Report itTake your temperature and call the team straight away.
Samples — Blood cultures, urine, swabs.2SamplesBlood cultures, urine, swabs.
Antibiotic — As soon as possible, without waiting for results.3AntibioticAs soon as possible, without waiting for results.
Fine-tuning — If a germ is found, treatment is matched to it.4Fine-tuningIf a germ is found, treatment is matched to it.
Infection found quicklyThe cause becomes clear.
PreventionHidden viruses kept in check.
Targeted treatmentThe right antibiotic or antiviral.

06Key infection tests

TestSampleWhyWhen
HBsAg, anti-HBcBloodHidden hepatitis BBefore treatment
Hepatitis C, HIVBloodHidden infection, choice of medicinesBefore treatment
CMV, HSV, VZVBlood (antibodies or PCR)Risk of the virus waking upBefore; regularly after a transplant
Blood cultureBlood (vein and line)Bacteria and fungiDuring a fever
Respiratory PCR panelNose and throat swabFlu, RSV, COVID-19 and othersCough, runny nose, fever
Galactomannan, beta-D-glucanBloodFungal infectionProlonged neutropenia
CRP, procalcitoninBloodTrend of inflammationSuspected infection and follow-up

* The list of tests varies with the centre, the type of treatment and the symptoms.

07Key terms

Screening
Looking for a hidden infection when there are no symptoms.
Reactivation
A virus hidden in the body becoming active again.
Blood culture
Finding germs in the blood by growing them.
PCR
A fast test that finds a germ’s genetic material.
Febrile neutropenia
A fever when neutrophils are low; an emergency.
Prophylaxis
Medicine given in advance to prevent an infection.
Galactomannan
A particle of mould that can be found in the blood.
Sensitivity test
A test showing which antibiotics a germ responds to.

08Key points

  • Hepatitis B, C and HIV are checked before treatment.
  • If a hidden virus is found, a preventive medicine is often given.
  • A fever with neutropenia is an emergency.
  • During a fever, blood cultures are taken from a vein and from the line.
  • Antibiotics start without waiting for results.
  • A PCR panel shows many viruses from one swab.
  • You can ask why each test is being done.

09Frequently asked questions

Why am I being tested for HIV? I am not at risk.

This is not personal suspicion but a rule for everyone. HIV affects the choice of some medicines and how infections are prevented. The result is confidential.

My anti-HBc is positive but I have never had hepatitis. What does this mean?

It can mean you met the virus in the past, often without symptoms. To keep the virus from waking up during treatment, your doctor may monitor the viral DNA or give a preventive medicine.

My blood culture was negative but the antibiotics continue. Why?

In many infections, no germ is found in the blood. With neutropenia, antibiotics usually continue until your condition, temperature and blood counts improve. The exact length is decided by the team.

Someone in my family has flu. Do I need a test too?

If you have symptoms, tell the team; they will decide on testing or prevention. It protects you when family members are vaccinated and avoid close contact while they are ill.

Can I do the tests myself at home?

Checking your temperature at home is important, but infection tests and their interpretation should be handled by the team. If a home rapid test is positive, tell the team straight away.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. Why am I being tested for HIV? I am not at risk.

  2. My anti-HBc is positive but I have never had hepatitis. What does this mean?

  3. My blood culture was negative but the antibiotics continue. Why?

  4. Someone in my family has flu. Do I need a test too?

  5. Can I do the tests myself at home?

My questions

12Sources

  1. National Cancer Institute (NCI). Infection and Neutropenia during Cancer Treatment
  2. American Cancer Society. Infections in People with Cancer
  3. MedlinePlus (U.S. National Library of Medicine). Blood culture
  4. MedlinePlus (U.S. National Library of Medicine). Hepatitis Testing
  5. MedlinePlus (U.S. National Library of Medicine). HIV Screening Test
  6. MedlinePlus (U.S. National Library of Medicine). Cytomegalovirus (CMV) Tests
  7. MedlinePlus (U.S. National Library of Medicine). Chickenpox and Shingles Tests
  8. MedlinePlus (U.S. National Library of Medicine). Respiratory Pathogens Panel
  9. MedlinePlus (U.S. National Library of Medicine). Fungal Culture Test
  10. MedlinePlus (U.S. National Library of Medicine). Procalcitonin Test
  11. NHS. Hepatitis B
  12. Infectious Diseases Society of America (Clinical Infectious Diseases, 2011). Clinical practice guideline for the use of antimicrobial agents in neutropenic patients with cancer: 2010 update
  13. ESMO (Annals of Oncology, 2016). Management of febrile neutropaenia: ESMO Clinical Practice Guidelines
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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