Target condition being diagnosed

Typhoid and paratyphoid (enteric) fever are diseases caused by Salmonella enterica serovar Typhi and Paratyphi A respectively. The more common typhoid is an important infectious disease in developing countries with over 22 million new cases worldwide, leading to an estimated 200,000 deaths (WHO 2002). South and South‐East Asia are the most affected areas of the world with an estimated annual prevalence of > 100/100,000 (Crump 2004). Typhoid and paratyphoid fevers are prevalent in low‐ or middle‐income countries with inadequate sanitation and hygiene, particularly regarding food, water, and disposal of human excrement. Despite advances in technology and public health strategies, enteric fever remains a major cause of morbidity in the developing world (Bhutta, 2006). Urbanisation, global warming, and traditional methods of water‐side living have created even greater demands for clean water in developing countries (UNICEF 2006). Both typhoid and paratyphoid are most common where standards of personal and environmental hygiene are low, and only to this extent are these diseases tropical (Gill 2009).

The Gram‐negative bacilli are transmitted by the faecal‐oral route when food or water contaminated with infected faeces is ingested. The most important reservoirs of infection are short‐term convalescent or chronic human carriers. Food handlers who are carriers are a particularly important source of transmission (Gill 2009).

The clinical presentation of typhoid and paratyphoid fever varies from a mild illness with a low‐grade fever, malaise and slight dry cough to a severe clinical picture with multiple complications including intestinal perforation (Ismail 2006). Toxic apathy, blanching 'rose spots' on the trunk, abdominal organomegaly, and diarrhoea are also associated with enteric fever, but the clinical picture is highly variable between geographical location and age‐groups. Typhoid and paratyphoid can present in many different and non‐specific ways, thus posing a diagnostic challenge for the health professional. Most enteric fever is diagnosed on clinical grounds and treated presumptively. As a result the diagnosis may be delayed or missed, while other febrile illnesses are being considered (Parry 2002).

There are a number of reasons why there is significant resistance of Salmonella enterica serovar Typhi / Paratyphi A to antimicrobials worldwide. Health professionals in the tropics overprescribe antimicrobials for many reasons, including cultural factors and patient expectation (Okeke 2005). The purchase of drugs such as antimicrobials from untrained vendors and unlicensed pharmacists are commonplace in the developing world (Larsson 2008). A major challenge is the inability to confirm diagnoses in resource‐limited settings where traditional laboratory methods of diagnosing typhoid and paratyphoid are not available. Health care workers are therefore reliant on their clinical skills to make an educated guess of the cause of illness, and/or prescribe an antimicrobial that targets several bacteria (Shetty 2008). This over‐treatment has contributed to increasing resistance to fluoroquinolones (eg ciprofloxacin) and multi‐drug resistance of Salmonella enterica serovar Typhi / Paratyphi A within endemic Asian countries (Chuang 2009).

Index test(s)

Simple, reliable, point‐of‐care rapid diagnostic tests (RDTs) for typhoid and paratyphoid (enteric) fever have been a long‐felt need of clinicians working in endemic areas (Jesudason 2006). The tests need to be suitable for use in remote areas with limited diagnostic facilities and relatively untrained staff. They should be designed to yield a simple 'positive/negative' result at thresholds pre‐set by the manufacturers, similar to a pregnancy test. These results should normally be made available within 15 minutes, so that they can be used while the healthcare provider is dealing with suspected patients (http://www.rapid‐diagnostics.org/). Finally, such tests must be made available at low cost for use in resource‐limited settings.

The lack of RDTs in areas without microbiology facilities means that the burden of enteric fever is underestimated worldwide (Parry 2002). RDTs could help rationalise antimicrobial treatment and thus contribute to tackling the problem of resistance in endemic areas (Bhutta, 2006). RDTs could be incorporated into clinical algorithms for patients with fever from endemic areas to help guide management.

Typhoid and paratyphoid RDTs comprise a heterogenous group of different methods and formats. RDTs have been applied to either blood or urine samples. Blood RDTs (using either venous and/or capillary samples) are more common than urine tests. These RDT products include test formats based on lateral flow, flow‐through, agglutination or solid phase methods (Pastoor 2008).

RDTs may detect antigens (components of the causative Salmonella organism) or antibodies (markers of the human's immune response to the antigen). The type of antibody class or immunoglobulin detected could be either Immunoglobulin‐M (IgM), which may be indicative of recent exposure, or Immunoglobulin‐G (IgG), which can indicate recent or previous exposure. Examples of commercial RDTs for typhoid and paratyphoid which have been undergoing evaluation in recent years include Typhidot^® , Typhidot‐M^® , and TUBEX^TM (Baker 2010).

New RDT developments are likely to take a serological approach, although the identification of novel antigens free of cross‐reacting materials and antigen pools is a major challenge (Baker 2010).

Alternative test(s)

The gold standard for diagnosing enteric fever has been culture of the Salmonella enterica serovar Typhi or Paratyphi A organism from either bone marrow or peripheral blood. The mainstay of diagnosis in clinical practice is a positive blood culture, although the test is only positive in 40 to 60% of cases, usually early in the course of the disease (WHO 2002). This lack of sensitivity may be due to the low number of bacteria circulating in the blood, or may be affected by prior antimicrobial therapy (Wain 1998). Bone marrow culture gives a higher culture‐positive rate, probably because the concentration of organisms is higher than in the blood, and may even yield a positive culture after antibiotic therapy has been started (Wain 2001). Bone marrow culture is positive in 80 to 95% of patients with typhoid and paratyphoid, even in patients who have been taking antibiotics for several days regardless of the duration of the illness (Parry 2002). Although bone marrow cultures are more sensitive, they are difficult to obtain, relatively invasive, and of little use in public health settings (Wain 2001). Even with sophisticated laboratories, confirming the diagnosis of enteric fever can still be difficult as samples of blood or bone marrow may still not show evidence of the disease despite a patient actually having typhoid or paratyphoid.

False negative blood cultures depend on numerous factors including: volume of blood sample taken; the type of culture medium used; and the length of the incubation period (Massi 2005). The sensitivity of blood culture is higher in the first week of illness (Parry 2002). Widespread antimicrobial availability and prescribing contributes to the low sensitivity of blood culture (WHO 2002). The ratio of blood to broth in preparing the blood culture could affect culture positivity rates (Parry 2002) and highlights the issues of the quality assurance of laboratories in endemic countries.

The Widal test (WT) is an example of a serological test. It detects agglutinating antibodies to lipopolysaccharide (LPS) (O antigen) and flagella (H antigen). It is still widely used for the serological diagnosis of typhoid (House 2001). In its original format the WT required both acute and convalescent‐phase serum samples taken approximately 10 days apart. More recently, the test has been evaluated for use as a single, acute‐phase serum sample (Saha 1996). In enteric fever, titres often rise before the clinical onset, making it very difficult to demonstrate the diagnostic four‐fold rise between initial and subsequent samples (Gill 2009).

The role of the WT is controversial because the sensitivity, specificity, and predictive values vary considerably among geographic areas (Parry 2002). Test results need to be interpreted carefully in the light of previous history of typhoid / paratyphoid and vaccination. Interpretation of the result is also greatly helped by knowledge of the background levels of antibodies in the local healthy population (House 2001). The widespread use of typhoid vaccines, and the large number of cases of repeated exposure to Salmonella species, are found to lower the specificity of the WT (House 2001). Several other diseases caused by non‐Salmonella organisms (eg malaria, dengue, brucellosis) have been shown to exhibit cross‐reactivity in typhoid‐endemic regions (Olopoenia, 2000). There is considerable variation in agglutinin levels among non‐infected populations. These levels are susceptible to change over time, and depend on the degree of endemicity (Parry 2002). Despite these shortcomings of both sensitivity and specificity the WT, both simple and inexpensive, is still widely used as a diagnostic test (Fadeel 2004).

Nucleic acid amplification tests (NAATs) for typhoid and paratyphoid diagnosis, such as polymerase chain reaction (PCR), are being explored. Theoretically, NAATs could amplify DNA from dead or unculturable bacteria, thus addressing the concern of poor culture positivity because of pre‐treatment with antimicrobials (Wain 2001). However, a novel three‐colour real‐time PCR technique has been found to have the same limitations as culture in terms of sensitivity, and deemed an unsuitable methodology for the routine diagnosis of typhoid and paratyphoid (Nga 2010). Methods combining culture and PCR methods have been also been explored (Zhou 2010). However, the use of NAATs in developing countries will most likely be limited in the medium‐term for reasons of cost (Olsen 2004).

Rationale

RDTs have the potential to be useful for clinicians working in resource‐limited settings in the tropics. Differentiating the common causes of the febrile patient by clinical criteria can be very challenging without the laboratory support for blood films, serology, or blood cultures (Bhutta, 2006). A diagnostic test in such settings must be cheap, simple to perform, able to deliver a quick result, and be both sensitive and specific (www.rapid‐diagnostics.org). Such a test should correctly identify true cases of typhoid and paratyphoid among febrile patients, ensuring prompt and typhoid / paratyphoid‐specific treatment, allowing the avoidance of broad‐spectrum medication that covers all common causes of fever. In many endemic areas, treatment for typhoid may be given to all patients with fever (Larsson 2008). Diagnosis of enteric fever by an RDT could reduce unnecessary prescription of antimicrobials, reduce drug expenditure, and limit the development of antimicrobial resistance.

The evaluation of RDTs in enteric fever is complicated by the lack of a suitable gold standard. The isolation and culture of Salmonella enterica serovar Typhi or Paratyphi A from blood or bone marrow is the available reference standard, but does not have 100% sensitivity (Baker 2010). The sensitivity and specificity of the RDT may be difficult to interpret in that it is possible that RDTs are more sensitive than blood and/or bone marrow culture.