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Understanding antibiotic resistance
Understanding antibiotic resistance

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4.3.1 Detecting the patient infection response

Some tests detect chemicals produced by the patient in response to infection – these are known as biomarkers. One example is the chemical procalcitonin (PCT) which is made in response to bacterial, but not viral, infections. It can easily be detected in a blood sample taken from the patient.

In the following video you will see how PCT tests can be used in clinical practice to reduce the unnecessary use of antibiotics. You can then practise interpreting PCT levels in Activity 6.

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Transcript: Video 5 How rapid diagnostic testing for PCT reduces unnecessary antibiotic prescribing.

MAGGIE
When you take into account farm use, GPs, and hospitals, it's estimated that 2/3 of antibiotic use is either highly questionable or totally unnecessary. And that's a tragic waste. So the most recent developments are in methods to cut down that waste, and that could ultimately save many lives. One of those methods is being introduced here at the Royal Hampshire hospital.
DR MATTHEW DRYDEN
Hello, Mrs. Preston.
MRS PRESTON
Hello.
DR MATTHEW DRYDEN
I'm Dr Dryden. We've popped by today to see how you are. Can you tell us what brought you into hospital?
MRS PRESTON
Yeah, I have difficulty breathing. I get very, very breathless.
DR MATTHEW DRYDEN
One of the reasons we've come along as the infection team is to try and decide if you need any antibiotics.
MRS PRESTON
I see, yes.
DR MATTHEW DRYDEN
We've got a special blood test, which is a relatively new development, which can distinguish between bacterial infection and viral infection or no infection at all. So I think we will do that on your samples, which will help us make a decision as to whether you need antibiotics or not.
MRS PRESTON
Right. Right. Okey doke.
MAGGIE
Yeah, yeah, lovely to have met you. Cheers.
DR MATTHEW DRYDEN
Maggie, this is the virology and immunology lab. This is the lab that we would bring Gloria's blood to. Downstairs, Gloria's blood is being cultured. Now, the interesting thing is that the blood cultures are going to take up to five days to grow bacteria. They may come up in 24 hours if we're lucky, if they're there. But this test we can do in two hours, which helps us decide whether she's got a bacterial infection or not, and whether we have to give her antibiotics.
MAGGIE
The test is looking for levels of a blood protein called procalcitonin, which rises during a bacterial infection, but not during a viral infection.
DR MATTHEW DRYDEN
This is a machine that does procalcitonin tests. And we've got a result coming out here now. So here's the readout for the test. And it gives us a very accurate representation of the procalcitonin concentration in the blood. It is less than 0.05. And that indicates, for this particular patient, there's no necessity to get antibiotics now.
MAGGIE
Dr Dryden closely monitors patients like Mrs Preston to make sure not giving them antibiotics is the right decision. And since introducing the system, this unit has cut down antibiotic use by half.
DR MATTHEW DRYDEN
To have a test like procalcitonin, which enables us with our clinical diagnosis to be sure that the patient has or has not got a bacterial infection is really useful.
MAGGIE
It's only being used in a handful of places so far. But the ultimate goal is to make this test economical enough to use in GP surgeries where 80% of antibiotics are prescribed.
End transcript: Video 5 How rapid diagnostic testing for PCT reduces unnecessary antibiotic prescribing.
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Video 5 How rapid diagnostic testing for PCT reduces unnecessary antibiotic prescribing.
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Activity 6 Interpreting PCT levels

Timing: Allow about 10 minutes

Figure 11 shows the levels of PCT – in micrograms (μg) of PCT per litre of serum – in a patient with an infection and receiving treatment over 3 days (72 hours).

Study Figure 11 carefully and then answer the questions below.

A line graph of how procalcitonin levels change in a patient with an infection.
Figure 11 Levels of procalcitonin in a patient with an infection.
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This figure comprises a line graph showing how procalcitonin levels change in a patient with an infection. The horizontal axis is labelled time (hours) and is marked from 0 to 72 hours in 12 hour intervals. The vertical axis is labelled [PCT] micrograms per litre and is marked from 0 to 5 in intervals of 1. At 16 hours there is a vertical orange dashed line which is labelled antibiotic treatment started. An arrow at 0 hours indicated the onset of infection. The levels of PCT are indicated with a blue line which slopes steeply upwards from the onset of infection to a peak of 5 micrograms per litre PCT at 16 hours. The line then slopes gradually downwards towards the horizontal axis ending below 1 microgram per litre PCT at 72 hours. At the top of the graph the region before antibiotic treatment started is labelled Assessing infection likelihood and severity. The region after antibiotic treatment is labelled Assessing effectiveness of treatment.

Figure 11 Levels of procalcitonin in a patient with an infection.
  1. How do the patient’s PCT levels change over the 72-hour period?
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Answer

The patient’s PCT levels rise from 0 at infection onset to 5 micrograms per litre after approximately 16 hours. When antibiotic treatment starts, PCT levels decline from 5 micrograms per litre to less than 1 microgram per litre after 72 hours.

  1. Does the patient have a bacterial infection? How can you tell?
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Answer

Yes, the patient’s PCT levels rise from 0 at infection onset to 5 micrograms per litre after approximately 16 hours. Elevated PCT levels indicate that the patient has a bacterial, rather than a viral, infection.

  1. Is the antibiotic treatment effective? How can you tell?
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Answer

Yes. Once, antibiotic treatment has started, the patient’s PCT levels decrease from 5 micrograms per litre to less than 1 microgram per litre, indicating that the patient’s infection has cleared and the antibiotic treatment is effective.

  1. How could measuring the PCT levels in a patient be used to reduce unnecessary antibiotic use?
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Answer

Decreasing PCT levels show that the infection is being effectively treated by the prescribed antibiotics. If PCT levels remain high after treatment, doctors could quickly prescribe an alternative treatment, reducing the need to take ineffective antibiotics for an extended period.