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?
The automated incubator scans the bottles every 10 minutes to see if they are positive, by measuring the level of acidity created by the CO2 being emitted from the living organisms. Staff are then alerted when there are positive bottles to take off. Different systems have different methods of detecting acidic shift (change in colour of an indicator disc in the bottom of the bottle, or changes in the way polarised light is refracted) but they are all looking for the logarithmic change in CO2 production.
Don’t call us...we’ll call you!
It is not necessary to ring the laboratory to check on the status of a blood culture and yet many doctors do. The laboratory will call out any relevant results when they are available. If they haven’t called yet then there is nothing to say about the blood culture. Ringing the laboratory to “chase” results takes up the time of the laboratory staff and delays the processing of samples. If there is no result on the computer then the result is not available and the laboratory will not give out an incomplete result as it can lead to mistakes in patient management. Be patient and wait for the result to come.
Once the blood culture is removed from the incubator a small amount of the fluid is taken out of the bottle and “Grammed”. The Gram film result is quick revealing red blobs and purple blobs, which to the trained eye can be identified as the most likely microorganism. Once identified the Microbiologist will telephone significant results and give management advice to the doctors looking after the patient. For example, the Microbiologist might say “the cause is a Gram-negative bacillus growing both aerobically and anaerobically”. This may appear to be pointless jargon but refer to basic bacteria identification to understand its significance to diagnosing and treating patients on the ward.
Standard identification methods require 24 hours growth on culture media to get a pure culture of the microorganism followed by a further 24 hours incubation to identify it using an analysis of how it metabolises various chemicals. However some laboratories are fortunate enough to have access to MaldiTOF (Matrix-assisted laser desorption ionisation Time Of Flight) which uses mass spectrometry and can identify most bacteria within 4 hours of the blood culture signalling positive on the automated incubator, knocking 2 days off the previous turnaround time!
It is fast, reliable, accurate, and cheap (5p per test) after the initial cost of the machine (which is huge >£100,000). I’m a bit biased, although I have no vested interested in any technology company, I do think MaldiTOF is one of the most important things to happen to microbiology within my career.
Where are my Sensitivities?
The blood culture is also inoculated onto a selection of culture media and antibiotic discs added for sensitivity testing. The antibiotic sensitivities usually take at least 24 hours and sometimes longer if the microorganisms are slow growing or uncommon. You’ve guessed it...by adding those clinical details on the form you can make the process quicker and more relevant! Just think how helpful it would be if the laboratory tested antibiotics which the patient has been started on...if you’ve told them, they will!
There are automated systems for performing antibiotic sensitivities but these are expensive and in my experience do not add much to the management of patients. Microbiologists make a prediction of the antibiotic sensitivities based on their knowledge of antibiotic resistance and the identification of the bacteria e.g. knowing Serratia marcesans will test resistant to most beta-lactams due to the chromosomal AmpC it carries means the choice of sensitive antibiotics is restricted to second line therapies.
A report is only released from the laboratory after the microorganism has been identified and the sensitivities have been completed (2-7 days). This is why it is imperative that any communication (conversation or documentation) with the Microbiologist is recorded accurately and in a timely fashion within the patient’s notes. The current lab-to-ward reporting systems do not allow for this to be electronic and the demand for service means it is impossible for most Microbiologists to actually write in each patient’s notes however the communication needs to be there to help anyone else asked to see and give advice on the patient.
So in summary:
- Any patient suspected of having a blood stream infection should have a blood culture taken, don’t wait for a temperature (see earlier blog)
- Blood cultures should be taken carefully to avoid contamination
- Clinical details are vital for the safe processing of blood cultures
- Clinical details are also important to laboratory staff to ensure they perform the right tests and results are specific to the patient
- Blood culture incubation is an automated process
- Positive blood cultures are phoned to the doctors looking after the patient, there is no need to ring the laboratory for the results