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Revista da Sociedade Br asileir a de Medicina Tr opical 3 8 ( 2 ) :1 9 6 -1 9 8 , mar-abr, 2 0 0 5
Comparison of the thick smear and Kato-Katz techniques
for diagnosis of intestinal helminth infections
Comparação das técnicas de sedimentação espontânea e Kato-Katz
para diagnóstico das helmintoses intestinais
Fred Luciano Neves Santos
1, Elúzio José Lima Cerqueira
1and Neci Matos Soares
1ABSTRACT
Th i s stu d y c o m p a re d th e e f f i c i e n c y o f Ka to - Ka tz th i c k sm e a r a n d th i c k sm e a r te c h n i q u e s f o r th e d i a gn o si s o f i n te sti n a l h e lm i n th s. Th e se n si ti vi ty o f th e th i c k sm e a r te c h n i q u e wa s h i gh e r th a n th a t o f th e Ka to - Ka tz m e th o d f o r th e d i a gn o si s o f a ll h e lm i n th s e x c e p t Sc histo so ma manso ni.
Ke y-words: Ka to - Ka tz. Di re c t e x a m i n a ti o n . Se n si ti vi ty. Sp e c i f i c i ty. Pre va le n c e .
RESUMO
O o b je ti vo d e ste tra b a lh o f o i c o m p a ra r a e f i c i ê n c i a d o s m é to d o s d e Ka to - Ka tz e se d i m e n ta ç ã o e sp o n tâ n e a p a ra o d i a gn ó sti c o d a s h e lm i n tí a se s i n te sti n a i s. A se n si b i li d a d e d a té c n i c a d e se d i m e n ta ç ã o e sp o n tâ n e a f o i su p e ri o r a q u e la e n c o n tra d a p e lo m é to d o d e Ka to - Ka tz p a ra o d i a gn ó sti c o d e to d o s o s h e lm i n to s, e x c e to p a ra Sc histo so ma manso ni.
Pal avr as-chave s: Ka to - Ka tz. Se d i m e n ta ç ã o e sp o n tâ n e a . Se n si b i li d a d e . Esp e c i f i c i d a d e . Pre va lê n c i a .
1 . Lab o r ató r io de Par asito lo gia Clínic a da Fac uldade de Far mác ia da Unive r sidade Fe de r al da B ahia, Salvado r, B A. This wo r k was suppo r te d b y the Fundaç ão de Apo io a Pe sq uisa do Estado da B ahia ( FAPESB ) . B r azil.
Addr e ss to: Dr. Fr e d Luc iano Ne ve s Santo s. Lab . de Par asito lo gia Clínic a/FF/UFB A. Av. B ar ão de Ge r e mo ab o , s/n. 4 1 1 7 0 -2 8 0 Salvado r, B A. Fax: 5 5 7 1 2 3 7 -2 2 5 5 .
e - mail: ne c i@ ufb a. b r
Re c e b ido par a pub lic aç ão e m 2 8 /1 0 /2 0 0 3 Ac e ito e m 1 5 /1 2 /2 0 0 5
COMUNICAÇÃO/COMMUNICATION
I n 1 9 5 4 , Ka to a n d Miur a in tr o duc e d a th ic k - s m e a r tec hnique for fec al examination3. Soon thereafter, Katz modified
and adapte d this te c hniq ue fo r use in fie ld studie s4. This
me tho d was ado pte db y WHO8 fo r quantitative and qualitative
diagno sis o f inte stinal infe c tio ns c ause d b y he lminths suc h as Asc a ri s lu m b ric o ide s, Tric hu ris tric hiu ra, hookworm and
Sc histo so m a m a n so n i, and has also been used in laboratory diagnosis8. Although the Kato-Katz method is widely used for the
dia gn o s is o f in te s tin a l he lm inths infe c tio ns, its r e po r te d effec tiveness varies. Garc ia et al2 observed that the Kato-Katz
method was more sensitive than the thic k smear tec hnique for diagnosing helminths2, while Martin & Beaver5 c onc luded that
the Kato thic k-smear tec hnique is reliable and prac tic al for the quantitative diagnosis of hookworm, Trichuris Trichiura and
Schisto so m e infec tions5. On the other hand, Engels et al1 showed
that the Kato-Katz method is not suitable for hookworm, protozoa and filariform larvae detec tion1; while another study c onc luded
that the Kato m e tho d has a lo w se nsitivity fo r ide ntifying hookworm eggs, and observed that hookworm eggs c ollapsed and disappeared shortly after the thic k smear had c leared9. Here,
we present a c omparative study of the Kato-Katz and thic k smear methods for detec tion of Asca ris lum b rico ide s, T. trichiura, hookworm and S. m a nso ni.
This study was based on 2 5 8 stool samples collected from school-children, aged 6 -1 4 years, from Salinas da Margarida County Public School ( Bahia State) , from January to June 2 0 0 1 . All samples were examined by the quantitative Kato-Katz and the qualitative thic k smear methods, ac c ording to standardized proc edures described elsewhere6. Three slides were prepared per person, and
egg-counts from Kato-Katz slides were conducted 2 4 hours after its preparation. The sensitivity and specificity of each method in isolation and of the two methods together were calculated using Epi-info ( version 6 ) software. The kappa statistic1 5 was used to
assess the c onc ordanc e of test results obtained with the two individual techniques, and McNemar’s test for paired proportions was used to compare the prevalences obtained with each of the individual methods, according to the normal theory test7.
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Sa nto s FLN e t al
wi th h e l m i n th s . T h e d i ffe r e n c e wa s m o s t e vi d e n t fo r ho o k wo r m, whe r e the r atio o f po sitive sample s ide ntifie d b y thic k sm e ar and Kato - Katz was 7 3 : 1 . Ho we ve r, c o m b ine d examination with the thic k smear and Kato-Katz tec hniques gave better results than when the thick-smear was used alone ( Table 1 ) . Compared to thic k smear, the sensitivity of the Kato-Katz method was 6 8 .2 % for T. trichiura, 7 0 .8 % for A. lum b rico ide s and 1 .4 % for hookworm, and the spec ific ity was 6 0 .2 % for T. trichiura, 8 4 .1 % for A. lum b rico ide s, and 1 0 0 % for hookworm ( Table 2 ) . However, the infec tions detec ted with these different tec hniques wer e no t nec essar ily the same, whic h means that even the sensitivity of these im pro ve d individual tec hniques is far from ideal, and health personnel should be aware that a person is not
to be c lassified as uninfec ted on the basis of a simple negative examination by one of these tec hniques c onduc ted in isolation. The Kappa statistic indicated that agreement between the Kato and thick smear methods was good for A. lum brico ides (
κ
= 0 .5 5 ) , and m ar ginal fo r T. tri c h i u ra (κ
= 0 . 2 7 ) and ho o k wo r m (κ
= 0 .0 2 ) . In addition, Mc Nemar’s test indic ated that direc t examination was more likely to correctly identify T. trichiura than the Kato-Katz method (χ
2 = 3.64, p = 0.056) , although the prevalences ofA. lum bricoides were not significantly different (
χ
2 = 2.526, p = 0.11) . However, c omparison of the individual methods with the McNemar test indicated that direct examination was more reliable for determining infection with T. trichiura. Larger sample sizes would be necessary for determining whether the direct examination is also more reliable for detecting A. lum b rico ide s.The present study found that the Kato-Katz method were less sensitive than thic k smear for detec ting hookworm infec tions. Pr e vio us r e po r ts1 9 have sugge ste d that Kato -Katz c o uld b e
adequate for detec ting hookworm. However, these studies only examined a single slide per patient, and the Kato-Katz slides were read immediately after preparation. It is likely that hookworm eggs deteriorate during the standardized 2 4 -hour period before slide reading, whic h would explain the low prevalenc es identified by Kato-Katz in this study. Nevertheless, we c onc luded that the Kato-Katz method, when used with the thic k smear, would be
Table 2 -Resu lts of thick smear an d Kato-Katz an alysis of 258 stool samples from pu blic school children in Salin as da Margarida Cou n ty, Bahia.
Thick smear
Kato-Katz positive negative sensitivity specificity Kappa Index
( %) ( CI) ( %) ( CI) ( CI)
T. trichiura positive 1 1 6 35 6 8 .2 6 0 .2 0 .2 7
( 6 1 .2 – 7 5 .2 ) ( 5 0 .0 – 7 0 .5 ) ( 0 .1 5 – 0 .3 9 )
negative 54 53
A. lum brico ides positive 85 22 7 0 .8 8 4 .1 0 .5 5
( 6 2 .7 – 7 9 .0 ) ( 7 8 .0 – 9 0 .2 ) ( 0 .4 5 – 0 .6 6 )
negative 35 1 1 6
Ho o k wo rm positive 1 0 1 .4 1 0 0 0 .0 2
( -.1 .3 – 4 .0 ) - ( -0 .1 7 – 0 .2 1 )
Negative 72 1 8 5
S. m a nso ni positive 1 1 1 0 0 9 9 .6 0 .6 6
- ( 9 8 .8 – 1 0 0 .4 ) ( 0 .0 1 – 1 .3 2 )
negative 0 2 5 6
Table 1 - Prevalen ce of common in testin al parasites detected by differen t techn iqu es of faeccal examin ation in 258 children from a pu blic school in Salin as da Margarida, Cou n ty, Bahia.
Prevalence ( %)
Parasite Thick smear Kato-Katz Thick smear +
Method technique Kato-Katz techniques
Helminthes,
Eggs/larvae
T. trichiura 1 7 0 ( 6 6 .0 %) 1 3 1 ( 5 8 .5 %) 2 0 5 ( 7 9 .5 %)
A. lum brico ides 1 2 0 ( 4 6 .5 %) 1 0 7 ( 4 1 .5 %) 1 4 2 ( 5 5 .0 %)
Ho o k wo rm s 7 3 ( 2 8 .3 %) 1 ( 0 .4 %) 7 3 ( 2 8 .3 %)
S. sterco ra lis 4 ( 1 .5 %) - 4 ( 1 .5 %)
E. verm icula res 3 ( 1 .2 %) - 3 ( 1 .2 %)
S. m a nso ni 1 ( 0 .4 %) 2 ( 0 .8 %) 2 ( 0 .8 %)
partic ularly appropriate for large-sc ale surveys bec ause of its simplic ity, lower c ost, and rapidity.
ACKNOWLEDGEMENTS
To Craig Andrews Milroy for his help with the c ritic al review of the manusc ript and for statistic al analysis.
REFERENCES
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