Doctors will frequently encounter patients with infections in their daily practice. Here is a brief outline of how we use clinical clues and laboratory tests to diagnose infections.
History
Despite having access to superb medical technology, don’t ever underestimate the importance of doctors talking to and examining patients. This is especially so in the field of Infectious Diseases where even a small clue from the distant past can yield a diagnosis in the present.
This is one of the attractions of this field – you are truly a detective, delving into histories and deciphering clues to unravel a mystery.
Travel history
Although living in an age of political correctness, some bugs are firmly xenophobic. In other words, some infections are only found in certain parts of the world. Therefore, knowing where a patient has travelled can be very helpful.
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| Travel history a must |
For example, a patient who travelled to Long Island (New York) some weeks earlier and now turns up with a rash and swollen joints may have Lyme disease.
Conversely, you can reassure someone who got bitten by a dog in Australia that they can’t get rabies because rabies isn’t found there.
Another reason for a detailed travel history is that some of these “geographical” infections can sit quietly in the body for years before they make you sick.
A case in point is meliodosis (due to a bug usually found in the soil), which can cause severe pneumonia in a person today even it was picked up years earlier from Northern Australia - if you don’t realize that he travelled to Northern Australia, then you might completely miss the diagnosis.
Sexual history
While this is a very personal aspect of an individual’s life, the simple truth is that many infections are transmitted through sex e.g. chlamydia, hepatitis B infection, gonorrhoea, syphilis, herpes and HIV to name but a few.
Furthermore, it is well recognised that during overseas travel, people are more likely to “experiment” than they would be at home, a phenomenon called “casual travel sex” (For those readers about to clobber their spouse who has recently travelled overseas, it is worth remembering that this is merely a pattern and doesn’t apply to everyone!). Therefore, it is vital to know a person’s sexual history, if for no other reason than to rule out a sexually transmitted infection.
Recreational
activities and pets
It often surprises people to hear that many seemingly innocent and everyday activities are potentially hazards for serious infections. Gardening is one example. A cut from a rose thorn can lead to sporotrichosis (a fungal infection) that can lead to a procession of lumps appearing on the affected limb. Potting mixes often contain Legionella, a bacterium that can cause pneumonia.
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| Doctor with a petri dish.
Pix courtesy getty images |
Recreational activities involving fresh water e.g. (swimming and white water rafting) can also be the source of numerous infections e.g. leptospirosis, schistosomiasis and Aeromonas skin infections.
During outbreaks of cryptosporidiosis (a parasitic infection), public swimming pools are often the major source of outbreaks.
Even contact with animals, including one’s own pets, can be a source of unusual infections. Parrots can cause pneumonia in humans - an infection called psittacosis (In fact, one case of psittacosis supposedly occurred when the owner performed mouth-to-mouth resuscitation on his unconscious pet bird! However, such intimate contact is not required to catch this infection).
A complicated skin infection in people in contact with fish raises the possibility of Mycobacterium marinum disease.
Therefore, a detailed history regarding recreational activities and animal contact can be very useful.
Diet
In certain patients, a dietary history can be important. A person with an unexplained fever and a history of consuming unpasteurised dairy products may well have brucellosis. Meningitis in pregnant women or
the elderly with a history of eating soft cheeses makes listeriosis more likely.
The Labor atory
Once doctors have made a clinical diagnosis, then they will arrange for tests to confirm that diagnosis. In the Microbiology Lab, the most common tests used for diagnosis are cultures, antibody tests and PCR. There are other tests too, but they are beyond the scope of this article.
Cultures
Microbiologists and Infectious Diseases Physicians are like diplomats in that we deal with different cultures everyday. Unlike diplomats, however, the cultures we deal with usually involve blood, urine and sputum (although one can only imagine what happens during heated debates at the UN Security Council). In clinical practice, cultures are mainly used to grow bacteria and fungi with only highly specialized labs performing cultures for viruses.
Any bodily fluid or part can be cultured. Culture involves taking a specimen (e.g. blood or a urine sample) and placing a small amount in a round petri dish filled with a jelly-like substance. This jelly-like substance is “culture medium” which contains nutrients for bacteria or fungi that allows them to grow rapidly. Usually within 48 hours, the bugs will have multiplied to such an extent that they will be visible on the culture plates as multiple dots.
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| A minor accident when gardening can lead to infection |
It is typically at this point that the laboratory staff will be able to perform further tests on the bug to determine both its identity and the antibiotics that will be effective against it.
This is the reason why your doctor can’t give you an instant result from your culture: it takes time for the bugs to grow in the lab before the lab staff can test them.
It is worth knowing that there are many kinds of culture media for different types of bacteria and fungi. Knowing which culture media to use for which patient is part of the expertise involved in the practise of Microbiology.
Also, while most bacteria and fungi will be cultured within a couple of days, some bugs can take much longer to grow. For example, the cause of tuberculosis, Mycobacterium tuberculosis, can take up to eight weeks to grow in culture!
Antibody tests (Serology)
The human body has a complex immune system that is designed to protect us from various external threats. One component of this is the production of antibodies. These are proteins that play an important role in controlling infections. They appear once our body has been exposed to an infection and therefore their appearance in blood can be used to diagnose certain infections. Some examples where we use antibody tests to diagnose infections include HIV, viral hepatitis, glandular fever, dengue, syphilis and chikungunya.
Although commonly used, the use of antibodies is not as straightforward as it seems. Sometimes, an antibody test will be positive as soon as a patient becomes sick e.g. during hepatitis A infection. But other antibody tests won’t become positive till after the patient is better e.g. leptospirosis; therefore, in such patients it isn’t a useful test for making a quick diagnosis. Antibody tests can be also be difficult to interpret when an infection occurs more than once. For example, using antibody tests to diagnose repeated dengue infections can be difficult indeed. The other problem with antibody tests is that they can cross-react with other infections or even with our own bodies! This means that a positive antibody test may not really reflect infection with that bug.
Polymerase chain reaction (PCR) tests
PCR relies on molecular testing to identify parts of bugs. Since it only tests for part of a bug, it doesn’t really matter if the bug is dead or alive. This is an advantage PCR has over cultures, where the bugs have to be alive for us to identify them. This is especially useful if a patient got antibiotics before the tests were taken, which means that cultures are less likely to be positive.
The main disadvantage of PCR is that the running costs for a PCR lab are high and beyond the reach of many laboratories.
Conclusion
Despite all the tests available, taking a thorough history from a patient is vital to considering all the possible causes of an infection. Then there are a number of techniques available in the laboratory to confirm or exclude that clinical diagnosis and guide treatment of the patient.
(The writer is a Consultant Physician in Infectious
Diseases and Senior Lecturer at the Australian National University
Medical School.)
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