Abstract
Detection of Giardia and Cryptosporidium in clinical stool specimens using the ColorPAC and ProSpecT enzyme immunoassays revealed 98.7% agreement for Giardia detection and 98.1% agreement for Cryptosporidium detection. Sensitivities were uniformly 100%. The specificities of the ColorPAC immunoassay for Giardia and Cryptosporidium detection were 100 and 99.5%, respectively, and those for the ProSpecT assay were 98.4 and 98.6%, respectively. The false-positive reactions with the ProSpecT assay occurred with specimens that were grossly bloody.
Giardia lamblia is the most common intestinal parasite in the United States, infecting approximately 2.5 million persons per year (1, 9, 15, 16). Infection with this organism, which usually occurs through the ingestion of contaminated water, results in watery diarrhea, abdominal cramps, and malabsorption that may last from weeks to months (2, 18, 24, 30). Another important intestinal parasite, particularly for patients with advanced AIDS, which may be transmitted through contaminated water is Cryptosporidium parvum (3, 5, 14, 19, 25). Although sporadic disease is more common for these infections, both G. lamblia and C. parvum have also been shown to cause outbreaks involving day care centers (24, 31).
The traditional method for the detection of these organisms is a microscopic examination of stool preparations for the cysts and trophozoites of Giardia and for the oocysts of Cryptosporidium (12). This, however, requires a considerable amount of time and a medical technologist who is well-versed in the identification of fecal parasites (12, 13, 21, 22). Traditional methods may be insufficient to demonstrate the presence of organisms and when used nonjudiciously are costly (4, 6, 8, 11, 12, 21, 22, 33). Fortunately, over the past decade a variety of highly sensitive and specific enzyme immunoassays have been developed that detect the antigens of Giardia and Cryptosporidium (7, 10–13, 17, 23, 26, 27–29, 31). These have become widely accepted as methods for screening stools for the presence of Giardia and Cryptosporidium (6, 20, 25, 32). We have evaluated two commercially available enzyme immunoassays, the ProSpecT Giardia/Cryptosporidium Microplate Assay (Alexon-Trend, Inc., Ramsey, Minn.) and the ColorPAC Giardia/Cryptosporidium Rapid Assay (Becton Dickinson, Sparks, Md.), for the ability to detect G. lamblia and C. parvum, in archived and routinely obtained clinical stool specimens.
Archived and routinely obtained clinical stool specimens fixed in 10% formalin were tested using the ColorPAC Giardia/Cryptosporidium Rapid Assay and the ProSpecT Giardia/Cryptosporidium Microplate Assay. All of the stool specimens tested were patient samples fixed in both 10% formalin and polyvinyl alcohol; none of the specimens were spiked with organism. Two hundred and forty-one stool specimens were tested for the presence of G. lamblia and C. parvum. Forty-seven of the stool specimens tested were archived. The archived specimens were not more than 5 months from the date of collection. Tests were performed according to the manufacturers' guidelines. Stools were judged to contain the organism in question if both enzyme immunoassays (EIAs) were positive, or if one EIA was positive and a subsequent morphological examination revealed that organism. Stools were considered truly negative if both EIAs were negative, or if one EIA was negative and the subsequent morphological examination failed to reveal the organism in question. Modified trichrome-stained smears of polyvinyl alcohol-preserved stool and iodine wet preps from ethyl-acetate- or formalin-concentrated stools collected in 10% formalin were used to detect the cysts and trophozoites of G. lamblia, while a modified Ziehl-Neelsen stain from concentrated stool specimens was used to detect the oocysts of C. parvum (12). Microscopic examinations of the stool specimens were only performed if the EIAs were discrepant.
The results of testing are summarized in Tables 1 and 2. Fifty-three of the 241 stool specimens were positive by both assays for the presence of Giardia. Three stool specimens were positive by ProSpecT assay but negative by the ColorPAC assay; the morphological examination of these stools failed to reveal Giardia. One hundred and eighty-five of 241 specimens were negative for Giardia by both EIA methods. Nineteen of the 241 stool specimens were positive by both methods for the presence of Cryptosporidium. Three stool specimens were positive by ProSpecT assay but negative by the ColorPAC assay; these were the same three stool specimens that were positive for Giardia by the ProSpecT assay but negative by the ColorPAC assay. A single specimen was positive by the ColorPAC assay but negative by the ProSpecT assay. None of the specimens with discrepant EIA results were seen to contain Cryptosporidium upon morphological exam. The remaining specimens were negative for Cryptosporidium by both EIAs. None of the archived specimens were discrepant. All the assays demonstrated 100% sensitivity. The specificity of the ColorPAC assay was 100% for Giardia and 99.5% for Cryptosporidium. The specificity of the ProSpecT Giardia/Cryptosporidium assay was 98.4% for Giardia and 98.6% for Cryptosporidium.
TABLE 1.
Performance of the ColorPAC Giardia/Cryptosporidum Rapid Assay and of the Giardia/Cryptosporidium Microplate Assay
| Assaya and genus | No. of specimens for which results were:
|
Total no. of specimens tested | |||
|---|---|---|---|---|---|
| True positive | False positive | True negative | False negative | ||
| ColorPAC | |||||
| Giardia | 53 | 0 | 188 | 0 | 241 |
| Crypstosporidium | 19 | 1 | 221 | 0 | 241 |
| ProSpecT | |||||
| Giardia | 53 | 3 | 185 | 0 | 241 |
| Cryptosporidium | 19 | 3 | 219 | 0 | 241 |
ColorPAC, ColorPAC Giardia/Cryptosporidium Rapid Assay; ProSpecT, Giardia/Cryptosporidium Microplate Assay.
TABLE 2.
Sensitivities and specificities of the ColorPAC Giardia/Cryptosporidum Rapid Assay and of the Giardia/ Cryptosporidium Microplate Assay
| Genus | Result fora:
|
|||
|---|---|---|---|---|
| ColorPAC
|
ProSpecT
|
|||
| Sensitivity (%) | Specificity (%) | Sensitivity (%) | Specificity (%) | |
| Giardia | 100 | 100 | 100 | 98.4 |
| Cryptosporidium | 100 | 99.5 | 100 | 98.6 |
ColorPAC, ColorPAC Giardia/Cryptosporidium Rapid Assay; ProSpecT, Giardia/Cryptosporidium Microplate Assay.
In this study, we evaluated the ability of two commercially available assays, the ColorPAC Giardia/Cryptosporidium Rapid Assay and the ProSpecT Giardia/Cryptosporidium Microplate Assay, to detect the presence of Giardia and/or Cryptosporidium in archived and routine clinical stool specimens. We found the ProSpecT and ColorPAC enzyme immunoassays comparable, with 98.7% agreement for Giardia detection and 98.1% agreement for Cryptosporidium detection. Both assays were highly sensitive and specific. The reason for the false-positive reactions is uncertain. However, it was noted that all the specimens that generated false-positive results with the ProSpecT Giardia/Cryptosporidium Microplate Assay were grossly bloody; the blood was confirmed by microscopic examination of the formalin-fixed specimen. The fact that these specimens were bloody was merely an observation; the total number of blood specimens was not recorded, as that was not the goal of this study. The effect of gross blood in the stool on this assay requires further investigation. In our opinion, although a time study was not performed, the ColorPAC EIA was simpler to use, with faster time-to-results than with the ProSpecT assay, similar to that reported previously (13). For positive specimens, this was primarily because the ColorPAC EIA has individual lines of identity, one for Giardia and one for Cryptosporidium, while the ProSpecT combination Giardia/Cryptosporidium assay requires supplemental testing to determine which organism is responsible for the positivity. The ColorPAC EIA, however, is more expensive than the ProSpecT assay, based on manufacturer's retail price.
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