Once in the Lab...
The blood culture bottles have arrived safely (see earlier blog) and are loaded on an automated incubator, which uses that barcode you now know not to peel off! How long it takes to signal positive depends on the type of organism, some take longer than others. Routinely negative blood cultures are destroyed after 5 days. That is really unhelpful if your patient has a slow growing microorganism or possible infective endocarditis. Did you write those details on the request form? If you did then the blood cultures will be incubated for up to 14 days.

The clinical details also help the laboratory decide how to safety process the blood cultures after they have signalled positive. So if you forgot to add the possibility of a high risk specimen e.g. typhoid or paratyphoid, you risk exposing the biomedical scientist handling the positive culture to a potential pathogen...blood-borne viruses, tuberculosis, shigellosis, salmonellosis, E.coli O157, Neisseria meningitidis, brucellosis, etc. Surely, being too busy to add these clinical details is negligent to your colleagues in the laboratory?

Writing the last blog about positive blood cultures, it occurred to me that many healthcare staff are probably not aware of how blood cultures are processed in a microbiology laboratory and from the discussions I  have had with junior colleagues, knowing when and how to take them is confused too. There seems to be an understanding that you "take the blood", send it to the lab, they plate it on to agar and then when something grows they tell you what it is…. This is not what happens!…. But it might explain why blood cultures can be misinterpreted and why the lab is repeatedly phoned for the results.

What is a blood culture?
A blood culture is a sample of blood, usually venous, from which a microbiology laboratory will try and grow bacteria or yeasts. Arterial blood can be cultured, though patients rarely have arterial blood taken as venous samples are easier to take.

As I have eaten lots of Halloween Trick or Treat sweets, “Candi-daemia” seems an appropriate blog this week. I’d like to thank a colleague in Singapore for suggesting the topic of this blog; the management of candidaemia. At first glance it can appear a difficult situation to manage, but in reality it’s not too tricky as long as you follow some key steps in the management. Coincidentally I have managed two such patients just last week so the suggestion of the topic was very timely. Thank you.

Candidaemia is the presence of Candida spp. in blood, diagnosed with a positive blood culture. It should always be taken seriously as it is only very rarely insignificant. Candida spp. are usually opportunists, exploiting some underlying defect in the patients immunity to infection. The most common sources of infection are:
• Central Venous Catheters (CVC)
• Intra-abdominal infections, perforation or abscesses
• Urinary tract infections, particularly if prosthetic material is present e.g. catheters or ureteric stents
• Intravascular infections associated with prosthetic material e.g. infective endocarditis, vascular grafts

Make the diagnosis
Candidaemia should be considered in any patient with a serious abdominal problem or intravascular or urinary tract prosthetic material e.g. CVC or urinary tract stents, especially if they are immunosuppressed.

Strictly speaking Candida spp. are facultative anaerobes, they grow both aerobically and anaerobically. In practice they grow much better in the aerobic blood culture bottles but they are relatively slow growing. Most significant bacteria will grow within 24 hours of incubating a blood culture, but Candida spp. (a fungus) tend to take 48 hours and sometimes even longer. They can also be very tricky to spot in the Gram film as there is often not much microorganism present. One tip for spotting them in a Gram film is if you cannot see anything obvious at high power (x1000) then switch to low power (x100) and scan the film, the fungal hyphae will be easy to spot as a clump of regularly shaped Gram-positive material which resolves into the typical Gram-positive appearance when you switch back to high power.