Many of you would now be aware of the shift from culture (stool MCS) to gene-based stool testing (stool PCR) which has now become available under Medicare subsidy. While this has been an exciting development that promised greater accuracy for the detection of parasites in our patients, there remains limitations. One of the biggest is the fact that the PCR test is based on just one stool sample compared to the 3 day samples used in the culture test.
While this is rationalised, both by the pathology companies and some doctors, by higher test sensitivity and specificity, it flies in the face of our understanding about the irregular shedding of parasites i.e. the presence of the parasite in an infected individual’s stool can vary from nothing to severe, just day to day, therefore diagnosis must be based on several days of stool collection to account for this.
A practitioner I mentor, faced with several patients with negative PCR results but a clinical picture and other pathology results (raised eosinophils, impaired iron levels etc.) that strongly suggested the presence of parasites has been debating this with her shared care providers trying to encourage them to still refer patients for the stool PCR but performed over several samples.
She came across this article as a nice piece of supportive evidence Irregular shedding of Blastocystis hominis (Venilla et al 1999): ncbi.nlm.nih.gov/pubmed/9934969
While there are numerous other studies confirming the irregular shedding of most parasites this is a handy paper perhaps to use to strengthen the case for PCR stool tests performed over 3 days rather than 1. Let’s face it – it’s a big enough ask to get our patients to collect stool – we should really ensure we have optimised their chances of getting an accurate result!