Tag Archives: legionella

“I told you I was sick”

Patient: “I have just come back from Bali and I have a fever and joint pains.”

Doctor: “Not to worry, I think you might have Dengue. I will do a blood test now, and another one in a month’s time to see if I am right.”

Patient: “Just a month, oh, it can’t be too serious then…”

Doctor: “No, it’s just Dengue, and what’s more I have some good news for you, I have just got some results back from the laboratory, that other illness you had 6 weeks ago turned out to be Legionella after all.”

Patient: “I told you I was sick…”

Even in the molecular age, it is surprising how often acute and convalescent serological testing is still utilised in clinical practice. Acute and convalescent serology is still frequently used for Legionella diagnosis, Dengue and other arboviruses, Lyme disease, Leptospirosis, the list goes on…

There are several potential reasons for this:

  • Requestor is used to ordering acute and convalescent serology-This is what the requestor has been doing for the past 30 years. Why change now?
  • Requestor is not aware that other options are available.- The requesting clinician may not be aware of the other options that are available for that particular condition, e.g PCR, antigen testing, etc.
  • Requestor scared/unaware they are allowed to order more “exotic” tests.- I find this a particular challenge when dealing with requests from the community setting. “I didn’t realise we were allowed to request a PCR test for this condition.”
  • Alternatives perceived to be more expensive.- And they are to a certain extent, particularly for arboviruses, but less so for Legionella. But don’t forget that 2 sets of serology along with the time for labour, interpretation and reporting is not an insignificant cost by any means.
  • Laboratory continues to offer acute and convalescent serology.- The laboratory can be guilty too, of not placing appropriate restrictions on acute and convalescent serology. The lab managers and clinical microbiologists of today are often  from an era where acute and convalescent serology was a mainstay of “diagnosis”.

If you were building a microbiology laboratory from scratch, and compiling a schedule or menu of tests, I would like to think acute and convalescent serology would be very low on the priority list.

There may be Public Health reasons why you might want to (retrospectively) confirm the diagnosis with acute and convalescent serology, but if this is the case, then it should be requested (or approved by) by Public Health doctors.

However for the purposes of acutely diagnosing the patient, by the time you get the result, the patient has either recovered, or not recovered…

Better to carefully restrict access to acute and convalescent serology, and “push” requestors into trying to make a real time diagnosis with more modern tests. The patient will thank you for it…



“First do no Harm”

We need to remember that there is always a patient at the receiving end of a test result, and that, wherever possible, the result should have a positive bearing on the management of the patient.

However some results can cause potential harm to the patient. How can this be?

A classic example of this is the urinary antigen test for Legionella pneumophilia. At first glance, this test, easily obtained and giving a result within 20 minutes, seems to do the job just fine.

But all is not quite as it seems….

The urinary antigen test for L pneumophilia only picks up serogroup 1. What about all those other serogroups of L. pneumophilia? It also has a sensitivity of about 70-75%. So 25-30% of true L pneumophilias (serogroup 1) are being missed. And finally, in NZ at least, the vast majority of Legionella disease is caused by Legionella longbeachae, for which the urinary antigen test is no good whatsoever.

The big worry is that a negative urinary antigen test in a sick patient will lead to antibiotic cover for Legionella being discontinued, and I have seen this happen quite a few times over the years, even in the ICU setting. We see this happen because the recipient of the result often takes it for gospel, without considering the whole picture of both the result and the patient, the sensitivity, specificity, PPV, clinical picture etc.

With the increasing availability of PCR testing for Legionella, maybe the days of urinary antigen tests for Legionella pneumophilia serogroup 1 are numbered, in much the same way as rapid turnaround PCR for RSV and Influenza are gradually pushing out their rapid antigen equivalents for these infections.

It is always a concern when a laboratory test can potentially do more harm than good, and I think this sometimes happens with urinary antigen for Legionella.

We have to be brave enough to both embrace and introduce new tests, and at the same time get rid of the old…