|LETTER TO EDITOR
|Year : 2014 | Volume
| Issue : 2 | Page : 136-138
Microscopy versus enzyme linked immunosorbent assay test for detection of Entamoeba histolytica infection in stool samples
Srujana Mohanty1, Nisha Sharma2, Monorama Deb2
1 Department of Microbiology, All India Institute of Medical Sciences, Bhubaneswar, Odisha; Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi, India
2 Department of Microbiology, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi, India
|Date of Acceptance||25-Feb-2014|
|Date of Web Publication||12-Aug-2014|
Department of Microbiology, All India Institute of Medical Sciences, Bhubaneswar, Odisha; Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Mohanty S, Sharma N, Deb M. Microscopy versus enzyme linked immunosorbent assay test for detection of Entamoeba histolytica infection in stool samples. Trop Parasitol 2014;4:136-8
Infection due to Entamoeba histolytica is one of the most common parasitic infections world-wide, affecting about 50 million people and resulting in 40,000-100,000 deaths/annum.  Clinical manifestations include diarrhea, dysentery, and extra-intestinal invasive disease. The traditional method of diagnosing intestinal E. histolytica infection by microscopic examination of fresh stool samples is only 50-60% sensitive and can give false-positive results. This is because E. histolytica is microscopically indistinguishable from the morphologically identical non-pathogenic species, Entamoeba dispar and Entamoeba moshkovskii. , A correct diagnosis of E. histolytica infection is, however, necessary to avoid undue treatment for amebiasis of patients infected with the non-pathogenic species. In view of the inadequacy of the light microscopy for differentiating between E. histolytica and E. dispar/E. moshkovskii, the WHO/Pan American Health Organization/United Nations Educational, Scientific and Cultural Organization Expert Consultation on amebiasis in a joint statement stressed the urgent need to develop improved methods for specific diagnosis of E. histolytica infection in developing countries.  In this study, we aimed to compare the detection of E. histolytica in suspected cases of intestinal amebiasis by a commercially available enzyme linked immunosorbent assay (ELISA) kit detecting E. histolytica specific antigen as against detection by microscopy.
The study was carried out in the Department of Microbiology, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi, a 1531-bed tertiary care hospital in North India, from July 2012 to February 2013. Stool specimens from patients presenting to the hospital with the complaints of loose stools, diarrhea and/or dysentery or other gastrointestinal complaints, and received in the laboratory within 2 h of collection, were included in the study. Only one sample was received per patient. Microscopic examination for the presence of parasite (cysts and/or trophozoites of E. histolytica/E. dispar/E. moshkovskii) was performed by the standard saline-lugol method after formol-ether concentration technique.  All stool specimens (without preservatives) were subjected to antigen test as per the manufacturer's instructions, to detect the presence of E. histolytica-specific galactose adhesin with a commercially available ELISA kit (E. histolytica-II; Techlab, Inc., Blacksburg, VA, USA). A positive result was defined as an optical density reading of ≥ 0.05 after subtraction of the negative control optical density. Per manufacturers' data, the sensitivity and specificity of the kit ranges from 96.9% to 100% and 94.7% to 100%, respectively.
A total of 167 stool specimens were received of which 64 were from the out-patient department and 103 from admitted patients. Twenty-nine samples were from the pediatric age-group (≤14 years). The samples were semi-formed/loose in 94 cases, watery in 32, with mucus and/or blood in 26, and formed stools in 15 cases. [Table 1] shows the results of microscopy and ELISA test for the detection of E. histolytica infection. Microscopy detected 15 samples to be positive for E. histolytica/E. dispar/E. moshkovskii complex cysts and/or trophozoites, of which only nine (60%) were E. histolytica positive by E. histolytica-II ELISA. From the remaining 152 samples detected to be negative by microscopy, the ELISA test detected E. histolytica infection in 10 samples. Thus, the total frequency of E. histolytica infection in the samples under study was 11.3% (19/167) using the ELISA test. The sensitivity of microscopy in detecting E. histolytica infection was 47.3% and the specificity was 95.9%; the positive and negative predictive values were 60.0% and 93.4%, respectively. Frequency of infection was 12.3% (17/138) in adult patients and 6.9% (2/29) in pediatric patients. Other stool pathogens detected were cyst of Giardia lamblia in 10 (5.9%) samples, ova of Ascaris lumbricoides in 4 (2.4%), cyst of Endolimax nana in 2 (1.2%), and ova of Hymenolepis nana and cyst of Chilomastix mesnilii in one (0.6%) specimen each. All were cases of mono-infection except one, in which a mixed infection of G. lamblia with E. histolytica was detected. [Table 2] shows the distribution of parasites recovered in relation to the stool consistency. It was observed that parasite recovery was highest from formed stool (5/15, 33.3%) followed by watery (8/32, 25.0%), semi-formed (19/94, 20.2%) and sample with mucus and/or blood (4/26, 15.3%) [Table 2].
|Table 1: Results of microscopy and ELISA test for detection of E. histolytica infection in stool specimens (n=167) |
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|Table 2: Distribution of parasites in relation to stool consistency (n=167) |
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The study provides an assessment of the true prevalence of intestinal infection due to E. histolytica during the period of study in this tertiary care hospital in Delhi, as opposed to its apparent prevalence suggested by microscopy; the results indicate a moderate (11.3%) endemicity. The findings of our study are significant for clinical application. [Table 1] reveals that if the test results of microscopy alone were taken into account for pursuing treatment for E. histolytica infection, then (a) As many as 6 out of the 15 patients reported positive would be undergoing unnecessary treatment; and (b) as many as 10 of the 152 patients reported negative would be missing necessary treatment.
In an earlier study  conducted in another part of Delhi to assess the clinical and microbiological spectrum of human immunodeficiency virus infection/acquired immune deficiency syndrome cases with diarrhea, the prevalence of E. histolytica determined by using a commercially available Enzyme Immunoassay kit (trademark/manufacturer not mentioned) was observed to be less (4.86%; 7/144) than in the current study. In a study conducted at a South Indian tertiary care hospital,  E. histolytica antigen ELISA (E. histolytica-II; Techlab, Inc., Blacksburg, VA, USA) gave positive results in 29 (64.4%) out of 45 stool samples, which were reported positive for E. histolytica, E. dispar, or E. moshkovskii by microscopy or culture. This finding is similar to our study where E. histolytica-II ELISA was positive in nine (60%) of the 15 microscopy-positive samples, thus indicating that a significant proportion of E. histolytica/E. dispar complex microscopy-positive samples are in fact of non-pathogenic E. dispar/E. moshkovskii complex species, for which undue treatment should be avoided.
Other than ELISA, various researchers ,, have evaluated the polymerase chain reaction (PCR) as a diagnostic tool for detection of E. histolytica in stool specimens targeting either the small subunit ribosomal RNA gene or specific episomal repeats species in comparison with microscopy and ELISA. In one study,  analysis of 127 stool samples by microscopy examination demonstrated 27 (21%) samples were positive for E. histolytica/E. dispar complex. Of these, only two samples presented diagnostic fragment of E. histolytica (132 bp) in a multiplex-PCR used for detection of E. histolytica/E. dispar. Among the negative samples detected by microscopic examination, one sample was positive for E. histolytica by multiplex-PCR. Assay for detection of E. histolytica antigen (E. histolytica-II; Techlab, Inc., Blacksburg, VA, USA) was concordant with multiplex-PCR for all the three PCR-positive samples. Another study  by Solaymani-Mohammadi et al. on 1037 stool samples from asymptomatic cyst passers in Iran, also observed a 100% correlation between the results from the TechLab E. histolytica-II stool antigen kit and those from nested PCR. However, the study by Stark et al.  reported that the TechLab ELISA kit did not prove to be useful in detecting E. histolytica, as it failed to identify any of the E. histolytica samples which were positive by PCR. Other authors , (considering that the circumstances allow) have advocated the use of all methods, i.e. microscopy, culture, antigen detection technique and PCR, in combination and evaluation together with the clinical symptoms to be the best approaches for the laboratory diagnosis of patients with amebiasis.
Today it is necessary to differentiate E. histolytica from E. dispar and E. moshkovskii because patient does not need treatment if only the latter species are identified, whereas if E. histolytica is identified, patient needs urgent treatment. Hence, it is necessary to get reliable results by using different diagnostic methods including specific methods like ELISA.
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[Table 1], [Table 2]