Polymerase chain reaction testing for faecal parasites: risks and alternatives

In this short report, written on behalf of the Australian Society of Infectious Diseases (ASID) and endorsed by its council, we highlight recent changes to stool pathogen testing (particularly for parasites) within Australasian laboratories and alert clinicians to our concerns regarding result interpretation.

Since 2013, many laboratories in Australasia have changed the technique used for stool parasite detection from subjective, time-consuming microscopy to multiplex polymerase chain reaction (PCR), which can detect multiple enteric bacterial and parasitic pathogens. A turnaround time of under 3 hours increases efficiency and reduces costs.1 Five protozoa are generally included in multiplex PCR assay: Giardia lamblia, Cryptosporidium spp., Entamoeba histolytica, Dientamoeba fragilis and Blastocystis spp.1

ASID and the Royal College of Pathologists of Australasia (RCPA) have significant concerns regarding two parasites included in multiplex PCR assay — D. fragilis and Blastocystis spp. — as their role as putative gastrointestinal pathogens is controversial and unproven. Both D. fragilis and Blastocystis spp., which are of uncertain clinical significance and may be colonising flora, have been detected at much higher rates by PCR than by routine microscopy, with prevalence rates of 17% for D. fragilis and Blastocystis.2 Similar rates have been found in Australian laboratories.3 Children aged under 10 years are the main population affected by the significant increase in detection.2,3 To date the best evidence in children, a double-blind randomised controlled trial, showed no difference between treatment and placebo for dientamoebiasis.4 A second peak occurs at 30–40 years of age, presumably among parents of children who test positive for D. fragilis.2 This has resulted in increased consultations to medical practitioners, unnecessary use of antimicrobials, and anxiety and uncertainty for families. Symptoms are often falsely attributed to these organisms, leading to overtreatment.

The results of these tests as part of the multiplex have resulted in confusion for clinicians. To optimise the use of faecal multiplex PCR in clinical practice and to minimise unwarranted treatment and anxiety, we recommend that practitioners:

• do not request stool pathogen assessment (including multiplex faecal PCR) on formed stool samples;

• do not request specific testing for D. fragilis or Blastocystis spp.;

• should reflect the markedly increased sensitivity with unclear significance in their clinical interpretation of pathology laboratory reports of detection of these parasites;

• adhere to comments appended to the laboratory report regarding the significance of D. fragilis and Blastocystis spp. and avoid specific treatment and further testing; and

• discuss with a paediatric or adult infectious diseases specialist or medical microbiologist, if clarification is required.

To eliminate uncertainties, the RCPA has released guidelines (http://www.rcpa.edu.au/Library/College-Policies/Guidelines/Faecal-pathogen-testing-by-PCR.aspx) recommending that laboratories do not include D. fragilis or Blastocystis spp. within enteric multiplex PCR testing. Where laboratories continue to test and report such results, ASID and the RCPA recommend that laboratories add a comment regarding the uncertainty of the significance of these organisms.