Ira Shah and C.T. Deshmukh
From the Department of Pediatrics, Seth G.S.
Medical College & KEM Hospital, Parel,
Mumbai-400012, India.
Correspondence to: Dr. Ira Shah, 240 D, Walkeshwar
Road, Malabar Hill, Mumbai 400006,
India. E-mail:
[email protected]
Manuscript received: November 18, 2003, Initial
review completed: January 28, 2004;
Revision accepted: May 14, 2004.
We conducted this study to determine efficacy
of Parasight-F (an HRP-II antigen dipstick method to detect P.
Falciparum) in children. A total of 30 children were enrolled in
the age group of 2 months to 12 years whose peripheral smear
showed asexual forms of Plasmodium falciparum. All patients were
tested for presence of HRP-II antigen of Plasmodium falciparum
in their blood by the Parasight-F dipstick test by either an
EDTA sample or a finger prick blood sample. The sensitivity of
Parasight-F was 83.3%. However, the sensitivity of Parasight-F
to detect Plasmodium Falciparum in case of mixed Plasmodium (Vivax
+ Falciparum) infection was only 25%. Also, all patients less
than 6 months of age had a negative Parasight-F test. Parasitic
index, prior treatment with antimalarials or severity of
Falciparum malaria have no effect on the sensitivity of
Parasight-F test. We conclude that Parasight-F is an effective
tool for diagnosis of Plasmoduim falciparum malaria in children.
Keywords: HRP-II antigen, Parasight-F, Plasmodium
falciparum.
Plasmodium falciparum is the most dangerous
type of malaria, left untreated it can lead to fatal cerebral
malaria(1). Rapid detection of Plasmodium falciparum
parasite in a patient’s blood is required for prompt treatment. To
detect peripheral parasitemia on microscopic examination of the
blood smear requires observer expertise and depends on the
parasitic index and the timing of collection of blood. Therefore,
a test that is bedside, rapid and not observer dependent is
required for urgent diagnosis of Plasmodium falciparum
infection. This study was thus undertaken to detect the
sensitivity of Parasight-F (a dipstick qualitative test to detect
histidine rich protein II-HRP II antigen of P. falciparum)
in P. falciparum infected children.
Subjects and Methods
Thirty children in the age group of 2 months to
12 years with fever, splenomegaly and a positive peripheral smear
showing asexual forms of Plasmodium falciparum were
prospectively evaluated for presence of Plasmodium falciparum
histidine rich protein II antigen in their blood by the Parasight-F
dipstick test. Patients whose peripheral smear showed gametocytes
of Plasmodium falciparum were excluded from the study as
the test fails to detect Plasmodium falciparum infections
with only gametocytes in the blood(2). Parasitic index was
determined by counting 1000 RBCs on thin peripheral smear and
reporting the number of RBCs infected per hundred RBCs counted.
Either an EDTA sample or a finger prick blood sample was collected
for the dipstick test. As the HRP II antigen from the blood
adsorbs to the antibody immobilized on the strip, a solid pink
line on the test strip is formed suggesting a positive test.
Efficacy of Parasight-F to detect Plasmodium falciparum
malaria in children, in different age groups, in presence of mixed
Plasmodium (Vivax + falciparum) infection, with different ranges
of parasitic index and prior treatment with antimalarials was
evaluated.
Results
Out of 30 patients with microscopic falciparum
parasitemia, 25 patients had a positive HRP-II antigen detection
dipstick test. Thus the sensitivity of Parasight-F test was
83.33%. Four patients had presence of mixed Plasmodium infection (Vivax
+ falciparum) on the peripheral smear of which 3 patients (75%)
had a negative HRP II antigen dipstick test. Of the 30 patients, 3
patients were below 6 months of age with parasitic index ranging
from 1% to 8% (mean = 3.3%). All 3 of them (100%) had a negative
HRP-II antigen dipstick test. However, of these 3 patients below 6
months of age, 2 patients (66.66%) also had presence of mixed
Plasmodium infection (Table
I).
TABLE I
Sensitivity of Parasight-F test in children.
Age
|
Total no. of
patients |
Mean parasitic
index |
Range of parasitic
index |
Positive
Parasight-F |
2-6 months
|
3
|
3.7%
|
1-8%
|
0 (0%)
|
6 month-1 year
|
2
|
1.6%
|
1.2-4%
|
2 (100%)
|
1 year-5 years
|
13
|
2.8%
|
1-8%
|
12 (92.3%)
|
5 years-10 years
|
8
|
4.8%
|
1-20%
|
7 (87.5%)
|
10 years-12 years
|
4
|
1.5%
|
1-2%
|
4 (100%)
|
There was no significant difference in the
sensitivity of Parasight-F test in children above 6 months of age
and gender. Eleven patients in the study had received chloroquine
prior to testing. However, all 11 patients (100%) had a positive
HRP II dipstick test and Plasmodium falciparum parasitemia
on peripheral smear. Eleven patients had severe malaria including
9 patients with cerebral malaria and the HRP II antigen test was
positive in 9 patients (81.81%) with severe malaria including 8
patients (88.88%) with cerebral malaria.
The parasitic index ranged from 1% to 20% with
mean parasitic index being 3.1%. Two patients (6.6%) with a
parasitic index of 1%, 2 patients (6.6%) with a parasitic index of
2% and 1 patient (3.3%) with a parasitic index of 8% had a
negative Parasight-F test. Parasitic Index had no statistical
significance on the Parasight-F test.
Discussion
Plasmodium falciparum malaria is seen
predominantly in Africa, India, Brazil, Afghanistan, Sri Lanka,
Thailand, Indonesia, Vietnam, Cambodia and China(1). A significant
number of malarial patients may have a negative peripheral smear
for Plasmodium parasite leading to delayed diagnosis and
treatment(3).
Other tests such as polymerase chain reaction (PCR)
with a sensitivity of 95% and specificity of 99%(4) and ELISA for
detection of serum IgM & IgG against Plasmodium falciparum
with a sensitivity of 78.1% and specificity of 94.9%(5) are
available. However, they are expensive, time consuming and
available only in specific laboratories.
Parasight-F is a rapid, bedside qualitative
dipstick test that detects the HRP II antigen of Plasmodium
falciparum on the RBCs. It does not require any
instrumentation or expertise. Studies in adults have found the
sensitivity ranging from 88.96% to 96.5% and specificity ranging
from 97% to 99.1%(3,6-8). Our study also showed a sensitivity of
83.33%. However, there were no studies available to exclusively
depict the efficiency of Parasight-F in pediatric population.
Utility of Parasight-F is not reliable in patients less than 6
months of age and needs to be further analyzed. Also, whether
presence of fetal hemoglobin or maternal antibodies has a
confounding effect on the test needs to be determined.
Though Parasight-F is antigen specific for
Plasmodium falciparum and there is no cross-reactivity with
Plasmodium vivax(9), we found the efficiency of Parasight-F
would drop in presence of mixed Plasmodium infection though
Banchongaksorn, et al.(10) found the test positive in 41
patients with mixed infection. One may suspect human error in
species identification during smear examination. However, since 2
(50%) of these patients were also below 6 months of age, whether
mixed infection actually has an effect of Parasight-F sensitivity
needs further evaluation.
Parasight-F test may remain positive for 7-10
days even after treatment with antimalarials(6,11). All our
patients who were treated with chloroquine prior to testing had a
positive HRP-II antigen test but they also had peripheral
parasitemia. Thus, HRP-II antigen dipstick test may not be useful
to assess response of antimalarial therapy, as it is a qualitative
test.
Though Parasight-F test has been found to have
high sensitivity when parasitemia is more than 30-40
parasites/mL(12,13), it is also noted that sometimes high levels
of parasitemia (>1000 parasites/microliter) also give false
negative results but the underlying reason is not known(7). In our
study, we found that parasitic index had no effect on the
sensitivity of the test. Also, severity of malaria does not affect
the sensitivity of the test.
Acknowledgement
The authors thank Dr. Kshirsagar, Dean, Seth
G.S. Medical College and K.E.M. Hospital for giving permission to
publish this article.
Contributors: IS conducted the study,
drafted the manuscript and conducted the statistical analysis. CTD
prepared the study design, supervised the study and will act as
the guarantor of the study.
Funding: None.
Competing interests: None stated.
