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Routine diagnosis is usually performed by a simple smear in normal saline or iodine solution. Several forms of B. hominis can be found in stool specimens, i.e. vacuolar, multivacuolar, avacuolar, granular, ameboid and cyst. However, most laboratories recognize only the vacuolar form as the diagnostic stage since it can be easily distinguished from other protozoa. As the result, the prevalence determined by wet mount preparation may be underreported. In vitro cultivation methods have been used to enhance detection; however, the usefulness of these methods is still controversial. Currently, some researches have shown that in-vitro cultivation is more sensitive and specific in detection of B. hominis than the simple smear and concentration technique because it increases the size and number of the organism.
The trichrome staining is routine diagnostic method for detection of B. hominis. Even though it increases sensitivity from a simple smear, it is limited due to the difficulty of identification because of parasites' variation in form and size, variation in staining, the need of professional technician and complicated steps of staining.
This table shows detection of B. hominis in stool specimens by simple smear and trichrome staining compared with in-vitro cultivation. 102 of the samples were found to be culture positive. The sensitivity and specificity of simple smear are 16.7% and 94%, while the sensitivity and specificity of trichrome staining are 40.2% and 80.4% respectively. The positive predictive values of simple smears (54.8%) and of trichrome staining 47.1%) were lower than the corresponding negative predictive values (69.7% and 80.4%, respectively). Compared to the cultivation method, the sensitivity of trichrome staining showed no significant difference, while simple smears were significantly different.
Receiver Operating Characteristic curve or ROC curve is the curve plotted between sensitivity and (1-specificity). It is used to measure the efficiency of the diagnostic method. The area under the curves of the simple smear and trichrome staining are 0.55 and 0.60 respectively. Thus this method can possibly predict the positive outcome even though the difference of the area under the curves between trichrome staining and simple smears was not significant.
Genotypic characterization using PCR has been a useful tool for the study of such infection; however, PCR amplification using stool specimens is rather insensitive. One of the methods prior to PCR detection was cultivation. Although there was no significant difference between in vitro cultivation and trichrome staining, in vitro cultivation using Jones medium is recommended to study the prevalence of B. hominis infection because of its high sensitivity, convenience and simplicity for a large number of samples, especially for field studies. In addition,for trichrome staining to be performed and examined correctly, it needs experienced technicians.However, if the actual number and forms of B. hominis in fresh specimens need to be determined, trichrome staining should be done before cultivation. B. hominis is genetically highly variable, as shown by different molecular techniques, mostly PCR. The molecular study of B. hominis in stool specimens using these techniques will provide better epidemiological data and might explain its pathogenicity. However, PCR detection of B. hominis in stool specimens is rather insensitive. Thus the development of an efficient method for the isolation and extraction of DNA is required prior to PCR. In vitro cultivation is useful for the detection of B. hominis by light microscopy and also for molecular studies using PCR because of increased numbers of the organism available.
Summarized by Thirayost Nimmanon
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