I was recently asked how can diagnostic tests be made more sensitive.
Not long ago in episode 52 I discussed the limitations of diagnostic tests. It’s probably worth going back to that episode and listening again or reading the show notes.
Sensitivity is simply defined as the ability of a test to detect a true positive. We want our diagnostic tests to be highly sensitive so that when we receive a positive result we can be confident the result reflects the presence of disease. Likewise, we want our tests to be specific too. Specificity is the ability of a test to exclude a true negative. That means we want a negative result to really mean an absence of the disease. What we don’t want though are tests that are so sensitive that we cannot trust the result, or more accurately, we cannot interpret the result adequately in the context of the patient in front of us. For example, PCR for the diagnosis of clinical chlamydia as well as screening sexually active asymptomatic people as part of a sexual health screen is regarded as the gold standard. The problem is that PCR cannot be used for test of cure because it is too sensitive. It will detect the nucleic acid from dead bacteria that are yet to be removed by neutrophils and other cellular inflammatory mechanisms. Read More
I was recently asked if I thought there is much of a future in microbiology for a specialist microbiologist.
It’s an interesting question. I grew up in an era of clinical specimens being looked at microscopically and set up on culture plates. The beauty of microscopy and culture is that you get a feel for quantity and quality of the specimen and the pathogens grown. For example, seeing lots of pus and one or two types of bacteria microscopically was good information to have when reading the cultures. If the bacteria growing matched the bacteria seen in the Gram’s stain, then the likelihood of a true infection is greater, especially with the presence of pus. Of course, there are some exceptions, like when Clostridium perfringens, which causes gas gangrene elaborates an α-toxin, it destroys leucocytes so in a Gram’s stain from a patient with gas gangrene all you see are these magnificent looking Gram-positive bacilli and sign of pus anywhere. Read More
So tonight, I want to talk about one of my favourite diagnostic tests in microbiology. I want to talk about blood for culture.
When patients are critically ill with an infection, they may become bacteræmic, that is, they have bacteria in their blood. This is a little bit of a misnomer. In a normal human with a normally functioning immunological system, it’s not uncommon to find very small numbers of bacteria in the blood, even what we might regard as highly pathogenic bacteria. I can recall on two separate occasions in my training, two patients who had blood collected for culture because of protocol and not because they were necessarily very unwell who grew meningococci in their blood. Most medical practitioners get excited and pay attention when I call them and explain we’ve grown a meningococcus. On these occasions however, the referring doctors were surprised because their patients were so well. Read More
I’m still in the diagnostic series of Medical Fun Facts. Tonight, I’m talking about eye infections. Eye infections give me the heebie-jeebies. Just the thought of anything going wrong with my eyes fills me with anxiety. As I grow older and my sight gets longer I’m acutely aware of just how important our eyes are. Infections of the eye can have devastating consequences.
It’s probably worth describing some ocular anatomy first so we can orient ourselves. Read More
Protein immunoblots are probably better known as Western blots. The name is derived sort of from knowing that Northern blots detect RNA and Southern blots detect DNA, so the name Western blot was assigned to immunoblots that detect protein.
Western blots or protein immunoblots are used to detect antibodies in serum specimens. They were commonly used in diagnosing infections caused by the Human Immunodeficiency virus and the Hepatitis C virus. Read More