Monday, 22 January 2018 11:46

The Ova and Parasite Exam: Practicing Good Stewardship of a Complex Test

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International travelers and immunocompromised patients are at increased risk of diarrhea caused by parasites. The trend of recent increases in these populations means that diagnosis of diarrhea with parasitic etiology is an important task for the clinical laboratory. However, the test used for this purpose, the ova and parasite (O&P) exam, is time-consuming and complex, with limitations that may not be recognized by all clinicians. It is therefore imperative that the clinical laboratory finds ways to ensure efficient utilization of the O&P.


What is an O&P exam and why is it so complicated?

The O&P is a microscopic examination of stool for the presence of parasites comprised of a wet preparation and a permanently stained slide. Practically, it is performed on fresh or fixed stool which may be concentrated before slides are prepared. On each slide, 200-300 fields are manually examined for the presence of cysts and eggs using a 40× and/or 100× objective as necessary. A wide variety of organisms can be detected by the O&P, including amoebae (ex. Entamoeba histolytica/dispar), ciliates (Balantidium coli), flukes (Schistosoma), eggs from a variety of tapeworms (Diphyllobothrium) or roundworms (Ascaris), and other organisms. However, the sensitivity of a single O&P is poor due to intermittent shedding of some parasites and eggs. As such, three tests over a span of 10 days are often performed for each patient.

An image comparing miscellaneous plant material and hookworm eggs that shows how similar they look under the microscope Figure 1. Unidentified plant material (A) and a hookworm egg (B). Some plant material shares morphological characteristics with hookworm eggs and may be misidentified as such, especially if the microscopist has rarely encountered hookworm eggs (as is often the case in low-prevalence settings). Source.

In addition to variable parasite shedding, sensitivity and specificity of the O&P is dependent on the experience of the operator, as manual interpretation of results can be challenging (for example, some plant material may share morphological characteristics with hookworm eggs, see Fig. 1). Further complicating the use of this test is the low rate of positive results—typically less than 1% in populations with low parasite prevalence (such as the United States). While a low positivity rate might be good news for patients (nobody wants to have a parasite), it poses a challenge to maintaining technologist proficiency and training new technologists. It is not uncommon for only a subset of technologists in a laboratory to be truly proficient in performing O&P exams. Therefore, it is in the best interest of patients, clinicians, and the clinical laboratory to ensure that this test is being performed at the appropriate time and in the appropriate patient population.


When is an O&P appropriate (and when isn’t it)?

The classic case for an O&P exam is a patient presenting with diarrhea and a history of travel to an area where parasitic infections are endemic. The Infectious Diseases Society of America also recommends O&P testing for immunocompromised individuals with diarrhea. However, patients who have been in the hospital for more than 3 days should not have O&P testing performed as nosocomial parasite infection is exceptionally rare.

An important limitation of the O&P is that it has inadequate sensitivity for Cryptosporidium and Giardia despite these two organisms being the most common parasites among individuals in North America with no international travel history. In this case, a direct fluorescent antibody (DFA) or enzyme immunoassay (EIA)  test specific for Cryptosporidium and Giardia is typically suggested. This test not only has a high sensitivity for Giardia and Cryptosporidium but is also substantially easier to interpret than alternative methods (such as an acid-fast stain for Cryptosporidium).

Unfortunately, many physicians are unaware of which organisms are detected by O&P. For example, Cryptosporidium cannot be seen well on routine O&P and alternative methods such as DFA, EIA, or acid fast staining must be used for adequate detection. However, in a survey of physicians from five states across the US, 76% responded that a routine O&P is adequate for detection of Cryptosporidium. This suggests that there are opportunities for clinical laboratories to help physicians make informed decisions on when to order an O&P exam, ranging from “gentle reminders” of proper use listed in the test description to “hard stops” that require clinicians to get approval prior to ordering tests.



The gentle reminder: providing comprehensive test descriptions

A brief internet search for publicly available O&P test descriptions from clinical laboratories around the country revealed that most had explanations of testing limitations. Commonly, these indicated that the test should not be ordered on current inpatients with >3 days hospitalization or for patients without significant travel history to endemic areas. Some laboratories also prominently direct clinicians towards the DFA or EIA tests for Cryptosporidium and/or Giardia in the absence of travel history to areas where parasites are endemic. Providing this information is undoubtedly a step in the right direction, but we don’t know whether it has had an impact on ordering practices.


The “hard stop”: clinical decision support tools

Clinical decision support tools can present a “hard-stop” to restrict ordering of tests that must be acknowledged by clinicians. Two major studies evaluated the utility of clinical decision support tools to reduce the unnecessary ordering of O&P testing. In one study conducted at Memorial Sloan Kettering Cancer Center (MSKCC) the O&P ordering system was modified to require approval of an Infectious Diseases clinician on all orders for patients with a hospital stay longer than 3 days. This intervention was effective; the number of O&P tests decreased by 63%, and “non-compliant” testing (i.e. testing done on patients with more than three inpatient hospital days) was reduced by 58.7%. Correspondingly, the percentage of positive tests more than doubled to 3.8%. A similar clinical decision support tool was implemented by the Cleveland Clinic in which ordering a “non-compliant” O&P required calling the laboratory for an exemption (which was always granted). As at MSKCC, this intervention resulted in non-compliant testing being cut in half.


Going molecular

In the future, multiplex molecular testing may allow clinicians to sidestep the issue of whether to use an O&P entirely by having one test for all suspected cases of infectious diarrhea. In this way, a single test would be able to diagnose most patients, providing an answer quickly even if the physician’s initial guess regarding etiology was wrong. Currently, the largest FDA-cleared panel contains 22 different targets including viruses, bacteria and parasites. However, these systems have not yet been widely adopted, possibly because of high cost in comparison to O&P and uncertainty as to the clinical significance of molecular detection of each of the pathogens included on the panel. Their impact on clinical practice remains to be seen, but these tests are poised to supplant at least some O&P testing.



The O&P exam is an important but difficult-to-perform test. Unfortunately, with >25% of experienced technologists preparing for retirement, there is also likely to be a significant loss in experience performing it. Therefore, it is up to the clinical laboratory to help physicians be good stewards of this test either through providing information regarding test utility, actively restricting orders through clinical decision support tools, or providing state-of-the art molecular testing as an alternative.



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Last modified on Monday, 22 January 2018 12:04
Kenneth (K.P.) Smith

Kenneth (K.P.) Smith is a postdoctoral fellow in James Kirby’s laboratory at Beth Israel Deaconess Medical Center. His research interest is in development of drugs and diagnostics for multidrug-resistant Gram-negative bacteria.