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Three Part Question

In [a child with possible pneumococcal infection], is [immunochromatographic rapid urine antigen testing]a [reliable screening test to diagnose/exclude pneumococcal aetiology]?

Clinical Scenario

A 9-month old infant has been admitted with fever, cough, shortness of breath and poor feeding. He is tachypnoeic with bilateral crackles and occasional rhonchi. Initial management is started with a provisional diagnosis of bronchiolitis. Nasopharyngeal aspirate for respiratory syncytial virus turns out to be negative. Over the next few hours, he is noted to have high grade pyrexia with a gradual clinical deterioration. As the on-call specialist registrar in paediatrics, you are now worried about a possible bacterial aetiology. You decide to commence antibiotics after sending a sample for blood culture. A chest radiograph, full blood count and C-reactive protein level do not help to distinguish between a viral versus bacterial infection. You are aware that in an infant with bacterial pneumonia, the most common causative organism is Streptococcus pneumoniae. A colleague informs you that your hospital laboratory can perform a rapid immunochromatographic urine antigen detection test which is widely used for diagnosing pneumococcal infections in adults.

You are not sure about the diagnostic utility of this test in children. You decide to do a literature search and critically appraise the evidence.

Search Strategy

Medline (1966 to date) and OLDMEDLINE (1950–1965) searched by the PubMed interface on 31 December 2007.

Secondary sources
Cochrane Library, Best Evidence, Clinical Evidence: no relevant articles found.
"Pneumococcal antigen urine" with limits (English, Human, All Child: 0–18 years) yielded 72 results. Replacement of "pneumococcal" with "pneumococcus" did not broaden the search. Among the results, there were no systematic reviews relevant to the clinical question.

Studies on purely adult populations as well as those evaluating outdated methods of antigen detection (latex agglutination and counterimmunoelectrophoresis) were excluded. Two articles were discarded as they were preliminary studies and their data were included in corresponding definitive studies published later.

Search Outcome

Seven articles were found to be of direct relevance.10–16 (table). Three other studies provided data on the impact of nasopharyngeal carriage and other confounding factors likely to result in false positivity.

Relevant Paper(s)

Author, date and country	Patient group	Study type (level of evidence)	Outcomes	Key results	Study Weaknesses
Charkaluk et al, 2006, France	287 hospitalised children divided into 3 groups: (1) 112 potentially pneumococcal infection (2) 54 non-pneumococcal infection (3) 121 no infection	Cross sectional (level 1c)	ICT positivity, blood culture	Culture positivity in group 1, 39.3% (blood 12.5%, other body fluids 26.8%) ICT: sensitivity 100%, specificity 55.9% (44.1–67.7), PPV 59.5% (47.8–70.3), NPV 100%, test accuracy 73.2% (65.0–81.4) Useful for rapid exclusion but cannot be used for positive diagnosis of pneumococcal infections	Well designed study which assessed the validity of ICT among children with and without pneumococcal infections. Simple, clear and reproducible study methods.
Tzeng et al, 2006, Taiwan	1334 hospitalised patients (1243 adults and 91 children) with respiratory tract infections, including URTI, LRTI and "indistinguishable" groups	Cross sectional (level 1c)	ICT positivity, blood culture	Only 11 children (12%) had S pneumoniae isolates. ICT: sensitivity 100%, specificity 62% (p = 0.0783), NPV 100% (p = 0.0278) Pneumococcal infections and nasopharyngeal carriage can be excluded by a negative ICT. Test positivity has no practical significance	Predominantly adult study but gives separate data for paediatric subgroup. Study grossly handicapped by small paediatric sample size, lack of clear definitions and inadequate statistical methods. Conclusions to be interpreted with caution in view of limited sample size.
Esposito et al, 2004, Italy	155 children less than 5 years old with clinically suspected invasive pneumococcal disease. 200 controls without clinical features of infection	Case control (level 1c)	ICT positivity, blood culture	Sensitivity 100%, specificity 80.6%, PPV 14.7%, NPV 100% Positive urine test unhelpful to distinguish true pneumococcal infection from nasopharyngeal colonisation	Appropriate study design with excellent methodology. Confidence intervals not provided. Only 5 children (3.2%) of entire sample had true pneumococcal infection (blood and CSF cultures).
Neuman et al, 2003, USA	346 children less than 5 years old divided into 5 groups: (1) 24 pneumococcal bacteraemia (2) 62 focal consolidation (3) 110 febrile non-bacteraemic children with leukocytosis (4) 71 febrile non-bacteraemic children with normal WBC count (5) 79 no bacterial infection	Cross sectional (level 3b)	ICT positivity; either blood culture or capsular antigen positivity	In group 1, 4 children (26.7%) had positive blood cultures. ICT: Group 1, 86.7% positivity; with increase up to 100% when concentrated urine was used. Group 2, 37.1% positivity; with increase up to 88.2% when concentrated urine was used. Specificity 62.9%; reduction to 11.7% when concentrated urine is used. High sensitivity with a very low specificity. Statistically significant difference between positivity in carriers and non-carriers. Antigen positivity significantly higher in younger carriers	Recruitment of samples to group 1 was mostly based on capsular antigen detection and not positive blood culture. Methodology otherwise acceptable. Attempts to demonstrate the high false positivity of ICT.
Dominguez et al, 2003, Spain	131 children divided into 4 groups: (1) 15 pneumococcal pneumonia (2) 35 mycoplasma pneumonia (3) 40 healthy children with nasopharyngeal carriage (4) 41 healthy children without nasopharyngeal carriage	Cross sectional (level 3b)	ICT positivity; either blood culture or capsular antigen positivity	In group 1, 4 children (26.7%) had positive blood cultures. ICT: Group 1, 86.7% positivity; with increase up to 100% when concentrated urine was used. Group 2, 37.1% positivity; with increase up to 88.2% when concentrated urine was used. Specificity 62.9%; reduction to 11.7% when concentrated urine is used. High sensitivity with a very low specificity. Statistically significant difference between positivity in carriers and non-carriers. Antigen positivity significantly higher in younger carriers	Recruitment of samples to group 1 was mostly based on capsular antigen detection and not positive blood culture. Methodology otherwise acceptable. Attempts to demonstrate the high false positivity of ICT.
Michelow et al, 2002, USA	154 hospitalised children with LRTI Urine antigen tested in 56 consecutive children	Cross sectional (although study included 42 controls, urine antigen testing was not evaluated in this group) (level 2b)	ICT positivity, blood culture	Among 56 children, 14.3% had positive cultures and 55% had positive ICT. ICT:Sensitivity 88%, specificity 75% Positivity not influenced by age, duration of previous antibiotic therapy or nasopharyngeal colonisation	Main purpose of this study was to evaluate utility of PCR in comparison with culture, serology and urinary antigen excretion. Specificity of assay and assessment of impact of colonisation not reliable as study is underpowered by the limited sample size. Concludes that further larger studies are needed.
Dowell et al, 2001, China	88 children with radiologically confirmed pneumonia, 198 matched controls	Case control (level 4)	ICT positivity, blood culture	35% of cases and 34% of controls had positive ICT (not significant; p>0.2). Among cases and controls, colonisation associated with statistically significant test positivity (p = 0.001). Test is not likely to be useful in distinguishing children with pneumococcal pneumonia from those who are merely colonised	Bacterial aetiology of pneumonia not clear; only 1 out of 88 cases had a positive culture for pneumococcus. Sensitivity and specificity not quantified. Unable to comment on predictive value of test.

Comment(s)

With widespread use of the highly effective vaccine against Haemophilus influenzae, pneumococcus has emerged as the single most common bacterial agent causing lower respiratory infections, septicaemia and meningitis. Many of these infections result in considerable morbidity and mortality. In paediatric clinical practice, it is often difficult to distinguish pneumococcal respiratory infections from those due to viral aetiology. Culture of S pneumoniae from blood or body fluids remains the gold standard for diagnosis of invasive pneumococcal infections. However, a low sensitivity level and the considerable time lag for a report limits the use of this investigation as a reliable tool for decision making at initial presentation. Polymerase chain reaction (PCR) for pneumococcus is available in specialised centres, but the need for expensive equipment and skilled personnel precludes its use in routine clinical practice. In up to 4% of children attending the emergency department with fever without a focus, occult bacteraemia by pneumococcus has been demonstrated (Carstairs). Many of these children will progress to invasive pneumococcal disease (Alpern). Attempts to identify this subgroup based on full blood count, erythrocyte sedimentation rate and C-reactive protein level have met with only partial success. The availability of a reliable rapid diagnostic test thus assumes tremendous clinical significance. A rapid test with very high specificity will enable early initiation of targeted antibiotic therapy with benzyl penicillin. Conversely, a highly sensitive diagnostic test will, if negative, rule out pneumococcal infection with certainty. This will enhance the clinician’s confidence in decision making and limit antibiotic use to indicated cases only. In addition, a reliable screening test will provide a clue to the aetiology even when cultures are negative, as is often encountered in clinical practice. Early attempts at rapid diagnosis were based on the detection of capsular antigen in urine by latex agglutination and counterimmunoelectrophoresis. However, these tests are now considered outdated due to their limited sensitivity. Binax Now is a recently developed simple immunochromatographic membrane test (ICT) for rapid detection of pneumococcal C polysaccharide antigens in urine. A swab dipped in urine is inserted into the test device and the result is available in 15 min. Since its introduction in 1999 following United States Food and Drug Administration approval, several authors have attempted to elucidate the usefulness of this rapid diagnostic assay. Most studies have been carried out in the adult population and results do encourage its use in the rapid diagnosis of invasive pneumococcal infections (Gutierrez, Murdoch, Smith). In comparison, there have been limited studies on children. Evaluation of a new screening test by comparing it against an established gold standard diagnostic test is ideally done using a cross sectional study, with samples for both tests taken at the same time (Knottnerus). Most authors in our review have followed this strategy by comparing ICT against a positive blood culture (Charkaluk, Tzeng, Esposito Neuman, Michelow and Dowell). Esposito et al and Dowell et al have gone one step further by including a control group, which enhances the validity of their conclusions. Binax Now was tested in children with potential pneumococcal invasive disease as well as in healthy children. Data from the latter group were included specifically to evaluate the impact of nasopharyngeal carriage, recent vaccination and other potential confounding factors. Three additional studies have provided valuable data about the high nasopharyngeal carriage rate in developing countries, which leads to significant false positivity and reduces specificity to 60–80% (Navarro, Hamer and Adegbola). Two well designed studies with reproducible methodology by Charkaluk et al and Esposito et al have clearly brought out the very high sensitivity and negative predictive value of this test. Although Tzeng et al and Neuman et al have reached similar conclusions, the presence of methodological flaws, unclear definitions and failure to account for confounding variables weakens the validity of their observations. Other studies have yielded comparable results and provide additional supportive evidence (Dominguez, Michelow). All the studies have thus consistently demonstrated a strikingly high sensitivity and negative predictive value of nearly 100%. Many countries including the United Kingdom have introduced universal vaccination against pneumococcus, which will result in reduced disease prevalence. However, the impact of vaccination on test positivity remains to be seen. Recent vaccination is likely to cause a false positive result, as has been demonstrated by Navarro et al. This, in addition to the confounding effect of nasopharyngeal carriage, should be taken into account when interpreting a positive test result, which by itself does not imply infection with S pneumoniae. The nearly 100% sensitivity and negative predictive value of the test which have been demonstrated by all the studies across diverse geographic and ethnic groups mean that a negative urine antigen test practically rules out pneumococcal infection in all suspected cases. This makes it a rapid, convenient and reliable bedside test. The urine antigen test is now increasingly available in most centres. It is imperative that clear guidance be provided to clinicians on the appropriate use and interpretation of the test. This analysis attempts to provide a rational basis for the same. A negative test result will add powerful statistical certainty to the clinician’s decision making in settings where occult bacteraemia or early invasive disease is a possibility. The test may also serve as a useful adjunct in tailoring the duration/type of antibiotic therapy once it has been started empirically. In children with invasive disease where pre-treatment with antibiotics has resulted in a negative blood culture, a reliable and specific antigen test would help to delineate the aetiology. However, it must be borne in mind that based on the current state of evidence, a positive test result is unlikely to be helpful. Its significance will depend on the background prevalence of nasopharyngeal carriage, the impact of widespread immunisation and the consequent change in serotypes causing disease. Further large scale studies are necessary to clarify this. A multi-site study supported by the Global Alliance for Vaccines and Immunisation is currently underway, looking at the utility of Binax Now in diagnosing pneumococcal meningitis (PneumoADIP Surveillance and research.).

Editor Comment

95% confidence limits are mentioned in parentheses where appropriate; ICT, immunochromatographic membrane test; LRTI, lower respiratory tract infection; NPV, negative predictive value; PPV, positive predictive value; URTI, upper respiratory tract infection.

Clinical Bottom Line

A negative urine antigen test rules out pneumococcal infection with almost 100% certainty. (Grade A) A positive test has no practical significance as it can be due to causes other than true infection. (Grade B)

References

1. Navarro D, Garcia-Maset L, Gimeno C, et al. Performance of the Binax Now Streptococcus pneumoniae urinary antigen assay for diagnosis of pneumonia in children with underlying pulmonary diseases in the absence of acute pneumococcal infection. J Clin Microbiol 2004; 42 (10): 4853–5.
2. Hamer DH, Egas J, Estrella B, et al. Assessment of the Binax Now Streptococcus pneumoniae urinary antigen test in children with nasopharyngeal pneumococcal carriage. Clin Infect Dis 2002; 34 (7): 1025–8.
3. Adegbola RA, Obaro SK, Biney E, et al. Evaluation of Binax Now Streptococcus pneumoniae urinary antigen test in children in a community with a high carriage rate of pneumococcus. Pediatr Infect Dis J 2001; 20 (7): 718–19.
4. Carstairs KL, Tanen DA, Johnson AS, et al. Pneumococcal bacteremia in febrile infants presenting to the emergency department before and after the introduction of the heptavalent pneumococcal vaccine. Ann Emerg Med 2007; 49 (6): 772–7.
5. Alpern ER, Alessandrini EA, Bell LM, et al. Occult bacteremia from a pediatric emergency department: current prevalence, time to detection and outcome. Pediatrics 2000; 106 (3): 505–11.
6. Gutierrez F, Masia M, Rodriguez JC, et al. Evaluation of the immunochromatographic Binax Now assay for detection of Streptococcus pneumoniae urinary antigen in a prospective study of community acquired pneumonia in Spain. Clin Infect Dis 2003; 36 (3): 286–92.
7. Murdoch DR, Laing RT, Mills GD, et al. Evaluation of a rapid immunochromatographic test for detection of Streptococcus pneumoniae antigen in urine samples from adults with community acquired pneumonia. J Clin Microbiol 2001; 39 (10): 3495–8.
8. Smith MD, Derrington P, Evans R, et al. Rapid diagnosis of bacteremic pneumococcal infection in adults by using the Binax Now Streptococcus pneumoniae urinary antigen test: a prospective, controlled clinical evaluation. J Clin Microbiol 2003; 41 (7): 2810–13.
9. Knottnerus JA, van Weel C, Muris JW. Evaluation of diagnostic procedures. BMJ 2002; 324 (7335): 477–80.[Free Full Text] [Online](accessed 1 April 2008). http://www.preventpneumo.org/activities/surveillance_research/
10. Charkaluk ML, Kalach N, Mvogo H, et al. Assessment of rapid urinary antigen detection by an immunochromatographic test for diagnosis of pneumococcal infection in children. Diagn Microbiol Infect Dis 2006; 55 (2): 89–94.
11. Tzeng DH, Lee YL, Lin YH, et al. Diagnostic value of the Binax Now assay for identifying a pneumococcal etiology in patients with respiratory tract infection. J Microbiol Immunol Infect 2006; 39 (1): 39–44.
12. Esposito S, Bosis S, Colombo R, et al. Evaluation of rapid assay for detection of Streptococcus pneumoniae urinary antigen among infants and young children with possible invasive pneumococcal disease. Pediatr Infect Dis J 2004; 23 (4): 365–7.
13. Neuman MI, Harper MB. Evaluation of a rapid urine antigen assay for the detection of invasive pneumococcal disease in children. Pediatrics 2003; 112: 1279–82.
14. Dominguez J, Blanco S, Rodrigo C, et al. Usefulness of urinary antigen detection by an immunochromatographic test for diagnosis of pneumococcal pneumonia in children. J Clin Microbiol 2003; 41 (5): 2161–3.
15. Michelow IC, Lozano J, Olsen K, et al. Diagnosis of Streptococcus pneumoniae lower respiratory infection in hospitalized children by culture, polymerase chain reaction, serological testing, and urinary antigen detection. Clin Infect Dis 2002; 34 (1): E1–11.
16. Dowell SF, Garman RL, Liu G, et al. Evaluation of Binax Now, an assay for the detection of pneumococcal antigen in urine samples, performed among pediatric patients. Clin Infect Dis 2001; 32 (5): 824–5.