"New medicines and new methods of cure always work miracles for a while." – William Heberden – an 18th century British physician.
Are you allergic to penicillin? It’s the most commonly reported drug allergy in this part of the world, with one in 10 people saying they have an allergy to the antibiotic.
However, it has been estimated that just 10 to 20 per cent of those reporting penicillin allergy are truly allergic.
So what, you may ask?
Well it’s important that the term ‘penicillin allergy’ is correctly applied in order to avoid adverse effects or inappropriate treatment. And in an era of antibiotic resistance and few new drugs in the pipeline, it makes sense to identify those who are, in fact, not allergic to a very useful group of drugs.
We know that people with suspected but unverified penicillin allergy are likely to be treated with alternative antibacterials that are more likely to contribute to microbial resistance.
Alexander Fleming serendipitously discovered penicillin between the two World Wars. By early 1944, the new miracle drug was widely available, initially for Allied troops and then the civilian population.
Interestingly, research has shown that people with a history of suspected penicillin allergy spend more time in hospital and have poorer clinical outcomes than patients without such a history.
In a US study, those with a label of penicillin allergy were exposed to significantly more antibiotics such as fluoroquinolones, clindamycin and vancomycin and had higher rates of Clostridium difficile, MRSA and vancomycin-resistant enterococcus (VRE) infections than their non- penicillin allergic peers.
And evidence is beginning to emerge that the removal of incorrect labelling of penicillin allergy can decrease broad-spectrum antibacterial use and reduce length of inpatient stay, mortality and treatment costs.
There are basically two types of penicillin allergy based on the timing of appearance of symptoms: immediate or delayed. Immediate reactions have their onset in one to six 6 hours (usually within 60 minutes) after exposure to a dose of an antibiotic and often involve initial skin reactions such as urticaria (hives) or itching all the way to swelling of the throat and neck and full blown anaphylactic shock.
Skin rash
Delayed or non-immediate reactions usually occur several days after exposure to penicillin and involve a different type of immune system response. Typically people notice either a flat reddish skin rash or hives.
Is penicillin allergy more likely to affect certain people? Women and older people are at higher risk; and there is a link between frequent exposure to penicillin and the likelihood of an allergic reaction.
In a study of children, ranging in age from six months to 14.5 years, in whom delayed-onset skin rash after penicillin administration had been reported, the rash recurred in only seven per cent when they were re-exposed to penicillin. This may be explained by its similarity to the common skin rash seen in viral illness in kids.
There are three steps needed to verify a diagnosis of penicillin allergy: a detailed clinical history; skin testing; and in certain cases an oral provocation test where the person takes penicillin again orally. If both skin and oral tests are negative, then they do not have a true penicillin allergy.
The investigating doctor will be especially interested in the route of administration of penicillin and whether the symptoms of a presumed reaction were immediate or delayed.
Skin testing should be carried out in a specialist allergy centre that is equipped to deal with a severe allergic response. Ideally it is performed shortly after the suspected allergy rash is first noted. For people with negative skin test results, an oral drug challenge may be recommended. But it is not suitable for anyone who has had a severe, life-threatening reaction to penicillin.
Vaguely aware you have a penicillin allergy?
There’s an 80 per cent chance you have been incorrectly labelled.
mhouston@irishtimes.com