What is an antibiogram, and why shouldn't you choose a medicine from it yourself?

The laboratory sheet has reached you. It has a column of drug names, and next to each one is one of three words: “sensitive”, “intermediate” or “resistant”. It is natural to want to pick the drug marked “sensitive” and head to the pharmacy. This article explains where that sheet comes from, what it shows, and why reading it needs judgments that only a doctor makes.

Where does an antibiogram come from?

First a culture is done. MedlinePlus says the type of sample depends on the site of infection: blood, urine, sputum, a wound or the throat. The laboratory grows the bacteria from the sample in a dish so they can be identified. Most bacteria take one to two days, and some take five days or more, so the result does not come the same day.

Once the bacteria are identified, the doctor may ask for an antibiogram. The same bacteria are tested against several antibiotics to see which ones stop their growth. MedlinePlus describes the result like this:

• Sensitive: the drug stopped the bacteria growing or killed them, and it can be a treatment option.
• Intermediate: the drug may work only at a higher amount or more frequent use. That decision is your doctor’s alone; do not increase the amount yourself.
• Resistant: the drug did not stop growth and is not a suitable option.

Some laboratories write “susceptible, increased exposure” instead of “intermediate”. EUCAST uses this name for the middle category, and the York and Scarborough hospitals in the UK explain that the wording does not indicate reduced effect of the drug, but means there is a chance of success when exposure to the drug is increased.

How is the plate read?

One common method for an antibiogram is the disk method. You may remember the plate shown on the home page of this site; it is this same test. According to the EUCAST guide, the steps are:

1. The bacteria are spread on the surface of the agar with a swab, in three directions, so the surface is evenly covered.
2. Several paper disks, each containing one antibiotic, are placed on the agar. A 90 mm dish usually holds at most six disks, because the zones must not overlap. The drug spreads from each disk into the agar.
3. The dish is placed in an incubator: 35 degrees for about 18 hours.
4. The next day the bacteria have formed an even layer. Around a disk whose drug stopped growth, a clear circle remains. This circle is called the zone. Around a disk whose drug did not work there is no zone, and the bacteria grow right up to the edge of the disk.
5. The diameter of the zone is read in millimetres on a dark background, with a ruler or calipers.

At this point nothing has been classified yet. We only have a number.

Why is a bigger zone not necessarily a better drug?

EUCAST says the zone diameter must be converted to sensitive, intermediate or resistant using breakpoint tables. These breakpoints are set separately for each drug and each type of bacteria, and for some drug and bacteria combinations the guide has special reading rules.

It is like two exams with different pass marks: a score of 14 passes in one and not in the other. The number alone does not say which did better. Likewise, you cannot compare the zones of two drugs just by looking at their diameters; each number has to be read in its own table.

The size of the zone also depends on test conditions. The EUCAST guide says a higher concentration of bacteria makes the zone smaller. If the dish is left out for a while after the disks are placed, the zones come out larger than they should. Incubating beyond the set time lets bacteria grow inside the zone, and a wrong “resistant” result can come out. For this reason the laboratory runs the method without change and with exact timing; the guide also stresses that the method must be followed fully.

In short, a bigger zone does not necessarily mean a better drug. The number means something only next to the breakpoint for that drug and that bacterium.

And after that?

Even when “sensitive” is written next to a drug, the choice of medicine is not finished. York and Scarborough hospitals write that clinical interpretation must take into account the site of infection, the type of bacteria, the patient’s condition and how the drug is given. The laboratory gives only one answer: how these bacteria behaved with this drug in this test. Whether the same drug is right for your infection, in your body, and with your other conditions becomes clear through an examination and your doctor’s conversation with you.

So choosing a medicine from the sheet carries three risks. The drug may not suit the site of infection. It may not be compatible with your condition. And the CDC reminds us that the wrong drug can delay the right treatment and cause serious side effects.

What should we do with the antibiogram sheet?

• Keep the original sheet. Write down the sample date, the type of sample (urine, wound, blood and so on) and the name of the laboratory, and bring them along. The first-visit guide has the checklist.
• Ask your doctor: Where was the sample taken from? Does this result match my symptoms today? What does the sheet mean for the site of my infection?
• Do not buy medicine from the list, and do not suggest it to others. If you have medicine left from an earlier course, see What should we do with leftover antibiotics?.
• To prepare a sample, read the samples and cultures guide.
• The section on resistant and recurring infections explains how the work-up goes when several courses of medicine have not worked.

When not to wait for the result

A culture result takes several days, and sometimes you should not wait. If there is confusion, fast breathing, pallor, a rash that does not fade under pressure, or you suddenly get worse, look at the list of urgent signs and call 115, Iran’s emergency number.

A reminder

An antibiogram is a picture of one test, and it is not read apart from an examination and a conversation. This article is for information and leaves the decision about medicine to the doctor.