Diagnosis of infectious diseases

To diagnose infectious diseases, in-vivo and in-vitro diagnostic procedures involving radiation are applied. In-vivo procedures provide images of living organisms and are used to diagnose such diseases as tuberculosis or osteomyelitis. In-vitro techniques, using test tubes or culture dishes, are used to diagnose for instance malaria, Ebola or HIV. 

Every year, 13 million people die worldwide from infectious diseases, most of them in developing countries. The human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS), tuberculosis and malaria are the most common and significant of these illnesses. Globally, a total of 36.9 million people live with HIV, with two million newly infected in 2014.

Tuberculosis is with 11 per cent of all deaths the second greatest killer from a single infective organism. Some 1.5 million people died in 2013 from this disease, while malaria, next on the list, infected 198 million people and took 584,000 lives in the same year. Migration from low-income to higher-income countries further exacerbates the problem, as does the potential for co-infection with HIV/AIDS and the development of drug-resistant strains of tuberculosis.

Overall global healthcare costs from only lower respiratory infections (including pneumonia), HIV/AIDS and malaria amount to more than US$34 billion, ranking third after those for cancer and heart disease. 

How nuclear medicine helps diagnose infectious diseases

Both in-vivo imaging and in-vitro methods are part of the nuclear medicine tool-kit to diagnose infectious diseases. In-vitro techniques involving imaging and molecular laboratory tests help identify infections and manage drug resistance.

However, in-vivo methods such as radiolabelling white blood cells are still the gold standard technique for infection detection. This technique is based on the property of leukocytes (white blood cells) to migrate into infected areas to destroy bacteria. With this method, a sample of white blood cells is labelled with the medical radioisotope Technetium-99m and reinjected into the patient. The imaging of the areas to which the cells spread in the body – a movement called focal uptake – then allows the infected regions to be identified.

Nuclear medicine studies and magnetic resonance imaging are used in the diagnosis and follow-up of different diseases, such as osteomyelitis  (infections of the bone that may involve the entire structure down to the bone marrow); fevers of unknown origin and infected vascular prosthesis. The latter are bacterial infections that can occur during operations to replace or bypass with a graft damaged or diseased blood vessels.

Considered very challenging medical conditions,  all these infections can be caused by bacteria carried from a distant site through the bloodstream; through inoculation from direct trauma; a contiguous focus of infection; or sepsis following surgery. The diagnosis of osteomyelitis is not always obvious, and radionuclide procedures are frequently performed as part of the diagnosis.

Positron emission tomography can diagnose a variety of infections with a fairly high degree of certainty, for example large-vessel vasculitis; abdominal infections such as inflammatory bowel disease; and thoracic and soft-tissue infection. It is also useful in tumour-induced fever caused by Hodgkin’s disease; aggressive non-Hodgkin’s lymphoma; colorectal cancer; and sarcoma. In patients with fever of unknown origin, in-vitro or in-vivo labelled white-blood cells methods are of limited value because of the rather low prevalence of granulocytic processes in a clinical setting.

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