Informational Tables

Table 2.2 Fecal specimens for parasites: options for collection and processing^a

Option

Pros

Cons

Rejection of stools from inpatients who have been in hospital for >3 days

Data suggest that patients who begin to have diarrhea after they have been inpatients for a few days are symptomatic not from parasitic infections but generally from other causes

There is always the chance that the problem is related to a health care-associated (nosocomial) parasitic infection (rare), but Cryptosporidium and microsporidia may be possible considerations

Examination of a single stool specimen (O&P examination); data suggest that 40–50% of organisms present will be found with only a single stool exam; two O&P exams (concentration, permanent stained smear) are acceptable but are not always as good as three specimens (this may be a relatively cost-effective approach); any patient remaining symptomatic would require -additional testing^b

Some think that most intestinal parasitic infections can be diagnosed from examination of a single stool; if the patient becomes asymptomatic after collection of the first stool, subsequent specimens may not be necessary

Diagnosis from a single stool examination depends on experience of the microscopist, proper collection, and the parasite load in the specimen; in a series of three stool specimens, it is often the case that not all three specimens are positive and/or may be positive for different organisms

Patient with diarrhea and relevant travel history outside of the United States
or
Patient with diarrhea who is a past or present resident of a developing country
or
Patient in an area of the United States where parasites other than Giardia are found (large metropolitan centers such as New York, Los Angeles, Washington, DC, Miami)

O&P exam, Entamoeba histolytica/ E. dispar immunoassay; immunoassay for confirmation of E. histolytica; various tests for Strongyloides may be relevant (even in the absence of eosinophilia), particularly if there is any history of pneumonia (migrating larvae in lungs), sepsis, or meningitis (fecal bacteria carried by migrating larvae); the agar culture plate is the most sensitive diagnostic approach for Strongyloides.

The O&P exam is designed to detect and identify a broad range of parasites (amebae, flagellates, ciliates, Cystoisospora belli, and helminths). If exams are negative and symptoms continue, special tests for coccidia (fecal immunoassays, modified acid-fast stains, autofluorescence) and microsporidia (modified trichrome stains, calcofluor white stains) should be performed; fluorescent stains are also options.

Examination of a second stool specimen only after the first is negative and the patient is still symptomatic

With additional examinations, the yield of protozoa increases (Entamoeba histolytica, 22.7%; Giardia lamblia, 11.3%; and Dientamoeba fragilis, 31.1%)^c

Assumes the second (or third) stool specimen is collected within the recommended 10-day time frame for a series of stools (protozoa are shed periodically); may be inconvenient for patient

Examination of a single stool and an immunoassay (EIA, FA, and lateral or vertical flow cartridge). This approach is a mix. One immunoassay may be acceptable; however, immunoassay testing of two separate specimens may be required to confirm the presence of Giardia antigen. One O&P exam is not the best approach (review last option below).

If the examinations are negative and the patient’s symptoms subside, probably no further testing is required

Patients may exhibit symptoms (off and on), so it may be difficult to “rule out” parasitic infections with only a single stool and one fecal immunoassay. If the patient remains symptomatic, then even if two Giardia immunoassays are negative, other protozoa may be missed (the Entamoeba histolytica/E. dispar group, Entamoeba histolytica, Dientamoeba fragilis, Cryptosporidium spp., the microsporidia). Normally, there are specific situations where fecal immunoassays or O&P exams should be ordered. It is not recommended to automatically perform both the O&P and the fecal immunoassay as a stool exam for parasites. Depending on the patient's history and clinical symptoms, either the O&P exam or a fecal immunoassay may be recommended, but generally not both.

The laboratory will pool three specimens for examination; perform one concentration and examine one permanent stain.

Three specimens are collected by the patient (three separate collection vials) over 7–10 days; pooling by the laboratory may save time and expense.

Organisms present in low numbers may be missed due to the dilution factor once the specimens are pooled.

The laboratory will pool three specimens for examination; perform one concentration and examine three permanent stained smears.

Three specimens are collected by the patient (three separate collection vials) over 7–10 days; pooling by the laboratory for the concentration would probably be sufficient for the identification of helminth eggs. Examination of the three separate permanent stained smears (one from each vial) would maximize recovery of intestinal protozoa in areas of the country where these organisms are most common.

Might miss light helminth infection (eggs and larvae) due to the pooling of the three specimens for the concentration; however, with a permanent stain performed on each of the three specimens, this approach would probably be the next best option in lieu of the standard approach (concentration and permanent stained smear performed on every stool). Coding and billing would have to match the work performed; this may present some problems where work performed does not match existing codes.

The patient collects three stools but puts a sample of stool from all three specimens into a single vial (patient is given a single vial only).

Pooling of the specimens would require only a single vial.

This would complicate patient collection and very likely result in poorly preserved specimens, especially regarding the recommended ratio of stool to preservative and the lack of proper mixing of specimen and fixative.

Perform immunoassays on selected patients^d by FA, EIA, or rapid cartridge methods for Giardia lamblia, (G. duodenalis. G intestinalis), Cryptosporidium spp., and/or Entamoeba histolytica/E. dispar or Entamoeba histolytica

Would be more cost-effective than performing immunoassay procedures on all specimens; however, the information needed to group patients is often not received with specimens. This approach assumes that the physicians have guidance in terms of correct ordering options (see table below).

Laboratories rarely receive information that would allow them to place a patient in a particular risk group, such as children ,5 yr, children from day care centers (may or may not be symptomatic), patients with immunodeficiencies, and patients from outbreaks; performance of immunoassay procedures on every stool is not cost-effective, and the positive rate will be low unless an outbreak situation is involved

Perform immunoassays and O&P examinations on request^c: Giardia lamblia (G. duodenalis. G. intestinalis), Cryptosporidium spp. and/or Entamoeba histolytica/E. dispar group, or Entamoeba histolytica
A number of variables will determine the approach to immunoassay testing and the O&P examination (geography, parasites recovered, positivity rate, and physician requests). Immunoassays and/or O&P examinations should be separately ordered, reported, and billed.

This approach will limit the number of stools on which immunoassay procedures for parasites are performed. Immunoassay results do not have to be confirmed by any other testing (such as O&P examinations or modified acid-fast stains). If specific kit performance problems have been identified, individual laboratories may prefer to do additional testing. However, the fecal immunoassays are more sensitive than the O&P examination and special stains (modified acid-fast stains). Also, this may be considered duplicate testing and may not be approved for reimbursement unless specifically ordered by the physician.

This approach will require education of the physician clients regarding appropriate times and patients for whom fecal immunoassays should be ordered. Educational initiatives must also include information on the test report indicating the pathogenic parasites that will not be detected using these methods. It is critical to make sure clients know that if patients have become asymptomatic, further testing may not be required. However, if the patient remains symptomatic, then further testing (O&P exams) is required. Remember, a single O&P may not reveal all organisms present.
Present plan to physicians for approval: immunoassays or O&P examinations, procedure discussion, report formats, clinical relevance, and limitations on each approach.

Perform testing using multiplex panel options (parasites available in the panel may vary, depending on the system)

Depending on the parasites included in the panel, testing sensitivity and specificity will be better than results obtained with the O&P examination and/or special stains

Most of the current multiplex organism panels contain very limited parasite options; thus, many pathogens may not be considered in testing; other methods will need to be used in order to cover other pathogenic parasites; future panels are being developed to include more parasite options.

^aO&P, ova and parasite; FA, fluorescent-antibody immunoassay. See the following
^bCartwright, C. P. 1999. Utility of multiple stool specimen ova and parasite examinations in a high-prevalence setting. J. Clin. Microbiol.37:2408–2411.
^cHiatt, R. A., E. K. Markell, and E. Ng. 1995. How many stool examinations are necessary to detect pathogenic intestinal protozoa? Am. J. Trop. Med. Hyg. 53:36–39.
^dKehl, K. S. C. 1996. Screening stools for Giardia and Cryptosporidium: are antigen tests enough? Clin. Microbiol. Newsl. 18:133–135.