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False positive malaria rapid diagnostic test in returning traveler with typhoid fever
© Meatherall et al.; licensee BioMed Central Ltd. 2014
Received: 14 February 2014
Accepted: 26 June 2014
Published: 9 July 2014
Rapid diagnostic tests play a pivotal role in the early diagnosis of malaria where microscopy or polymerase chain reaction are not immediately available.
We report the case of a 39 year old traveler to Canada who presented with fever, headache, and abdominal pain after visiting friends and relatives in India. While in India, the individual was not ill and had no signs or symptoms of malaria. Laboratory testing upon his return to Canada identified a false positive malaria rapid diagnostic (BinaxNOW® malaria) result for P. falciparum with coincident Salmonella Typhi bacteraemia without rheumatoid or autoimmune factors. Rapid diagnostic test false positivity for malaria coincided with the presence or absence of Salmonella Typhi in the blood.
Clinicians should be aware that Salmonella Typhi infection may result in a false positive malaria rapid diagnostic test. The mechanism of this cross-reactivity is not clear.
Diagnosis of malaria relies on both rapid diagnostic tests and thick and thin film microscopy for confirmation. RDTs have the advantage that no significant expertise is required and the test can be performed at the point of care. Delays in reading the peripheral smear can mean that clinicians rely on the RDT for initiation of therapy and early management.
A 39 year old who travelled from Canada to India for two weeks developed acute onset of fever upon returning to Canada. He was born in India and lived there until the age of 27 at which point he emigrated to Canada. He returned to India on an emergent basis for a family member’s funeral. He did not take malaria prophylaxis nor did he take anti-malarial medication on this occasion but had on previous trips to India where he returned to on a yearly basis to visit friends and relatives (VFR). He was otherwise healthy and does not take any medications chronically. He denies any previous travel-related illnesses. He had not received typhoid vaccination prior to this trip or in the past.
Blood cultures were drawn on the day of presentation when Malarone® was initiated and were positive for Salmonella Typhi with intermediate susceptibility to ciprofloxacin (E-test), but susceptible to trimethoprim-sulfamethoxazole (TSX) and ampicillin (Vitek2, Biomerieux) based on Clinical Laboratory Standard Institutes testing guidelines . He was referred to the outpatient adult infectious diseases clinic for treatment with three days intravenous Ceftriaxone followed by two weeks of oral TSX with complete resolution of his symptoms. Follow up blood cultures on day 4 and 20 were negative. Unfortunately, three weeks after completing therapy, he developed recurrence of fevers and his blood cultures were again positive for S. Typhi on day 37 without evidence for gallbladder disease based on abdominal ultrasound. He was then successfully treated for a relapse of typhoid fever with two weeks intravenous Ceftriaxone through a home parenteral therapy program with a peripherally inserted central catheter.
Results of BinaxNOW® malaria RDT, peripheral blood smear, PCR, and blood cultures
Day after presentation
Peripheral blood smear
Auto-immune profile of patient
Rheumatoid factor < 10 kU/L (normal range 0-20 kU/L)
Rh blood type
Anti SS-A/Ro 60
Anti Ro 52
Persistent hrp 2 antigen (up to 27% at day 28) after successful treatment of malaria is well documented both in endemic settings and in returning travelers [6, 7]. Furthermore, false positives for hrp 2 have been documented for patients with RF and co-incident parasitic infections such as schistosomiasis and toxoplasmosis, as well as viral infections such as hepatitis C virus . Generally, the RDT has been reported to work effectively (84.2% sensitive and 99.8% specific) in a U.S. teaching hospital when compared to microscopy . In a study conducted in Tanzania with a related RDT (Paracheck®) which also relies on hrp 2, the adjusted odds ratio of obtaining a positive RDT with a negative peripheral blood smear examination was 3.82 (2.25-6.50, N = 169) in the presence of non-Typhi Salmonella bacteraemia, as opposed to 1.52 with any invasive bacterial disease . These data suggest that co-infection with sub-microscopic P. falciparum occur with Salmonella spp. bacteraemia commonly in endemic areas. It is not clear whether some of these are false positive RDT results due to an interfering substance associated with Salmonella spp. bacteraemia. It has been reported that up to three quarters of Salmonella bacteraemia patients have been shown to be either recently or concurrently infected with malaria . However, studies that rely on the hrp 2 antigen alone and not peripheral smears (thick and thin blood films) and PCR to determine recent malaria infection may over-estimate numbers due to false positives. That is to say, false positivity with malaria RDTs due to persistent hrp 2 antigenemia after recent malaria infection, interfering substances such as RF or another pathogen, or actual co-infection will over-estimate rates of malaria. Moreover, it is difficult to tease out these phenomena in malaria and typhoid co-endemic settings.
In our patient, we cannot rule out earlier P. falciparum infection that cleared with partial immunity in this individual who was born in India. However, we do note that the RDT turned negative on day 4 when blood cultures transiently cleared on ceftriaxone. Clinicians who order blood cultures in addition to malaria testing in returning travelers to a non-endemic setting who present with fever, should be aware of possible false positive reactions in patients with Salmonellosis (typhoidal and non-typhoidal). It is not clear whether the Salmonella retains an antigen that mimics hrp2 or whether antibodies from the septic patient create false positive reactions through anti-idiotypic antibody reactions. Interestingly, the patient also had positive IgG and IgM to dengue virus during this episode, suggesting the individual was mounting a broad antibody response to several pathogens, possibly non-specific in nature as he had no classic dengue-like symptoms and PCR testing for dengue virus was negative. Of note, when the RDT is performed, whole blood (EDTA tube) is used containing the plasma where interfering antibodies reside. We conducted a retrospective analysis of the all RDT (n = 4514) performed on malaria suspected patients from 2008 - 2012 in our laboratory and found no other false positive T1 bands co-incident with S. Typhi bacteriemia. Therefore, the phenomenon observed with this patient appears to be a rare event. The possibility of a faulty BinaxNOW® lot was ruled out since each of the three positive results was from different lot numbers (057580, 061868, and 061318). This case reinforces the importance of expert microscopy and/or PCR in the confirmation of presumptive results with the RDT. It has been demonstrated previously that expert microscopy and PCR have superior analytical sensitivity to RDTs using latent class and receiver operating curve analysis . A limitation of our study was that PCR was not performed on subsequent specimens from the same patient per our current testing algorithm in order to save costs. However, the likelihood of a positive PCR result on subsequent specimens is unlikely because the excellent sensitivity of our PCR assay . We recommend that diagnosticians be aware that Salmonella bacteraemia may cause a false positive reaction in VFR and returning travelers who present with febrile illness of unknown etiology, and in surveillance studies conducted in endemic regions. False-positives can lead to unnecessary administration of potentially harmful drugs to the patient and misdiagnosis in surveillance of malaria if used as the sole diagnostic modality.
Written informed consent was obtained from the patient for publication of this Case report. A copy of the written consent is available for review by the Editor of this journal.
We thank the expert technical assistance of the Parasitology section at Calgary Laboratory Services.
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