Comment
Urine lipoarabinomannan testing in children with tuberculosis An estimated 1 million (999 792; 95% CI 937 877–1 055 414) children develop tuberculosis every year; most cases arise in southeast Asia (397 040; 350 615–447 474), Africa (279 825; 250 187–308 717), and the western Pacific (179 515; 159 246–202 626).1 Tuberculosis is probably a major unrecognised cause of under-5 mortality in tuberculosis-endemic areas, especially in children dying from pneumonia, meningitis, sepsis, HIV/AIDS, and severe malnutrition.2 Autopsy studies from five African countries identified tuberculosis in roughly 10% of 811 children (both HIV-positive and HIV-negative) who died from presumed pneumonia.3 This finding provides a sad reminder of the diagnostic challenges in resource-limited settings, which restrict children’s access to tuberculosis treatment that is cheap, effective, and well tolerated. The holy grail of tuberculosis diagnostics is a rapid and reliable point-of-care test that is able to differentiate latent infection from active disease and can be done on a readily available specimen.4,5 The tuberculin skin test provides information about likely Mycobacterium tuberculosis infection, but fails to identify active disease. Interferon-gamma release assays (eg, QuantiFERON-Gold In Tube [Cellestis, Carnegie, VIC, Australia] and T-Spot.TB [Oxford Immunotec, Oxford, UK]) provide improved specificity compared with the tuberculin skin test, but also fail to make the crucial distinction between latent infection and active disease.6,7 Although these tests are helpful to identify M tuberculosis infection in asymptomatic children during immigrant screening or contact investigation, their utility is restricted in tuberculosis-endemic settings where M tuberculosis infection is ubiquitous.7 Sputum smear microscopy, the traditional cornerstone of tuberculosis diagnosis, has no value in young children who are unable to expectorate. Induced sputum and gastric or nasopharyngeal aspirates provide alternative respiratory specimens,8 but specimen collection is cumbersome and the bacteriological yield is low, even with the use of liquid culture or Xpert MTB/RIF.9,10 In this issue of The Lancet Global Health, Mark Nicol and colleagues11 report the diagnostic accuracy of two commercially available rapid urine tests to identify active tuberculosis in children. Lipoarabinomannan is a unique component of the mycobacterial cell wall that is excreted www.thelancet.com/lancetgh Vol 2 May 2014
in the urine of some patients with tuberculosis. Findings of studies to assess the diagnostic ability of urine lipoarabinomannan assays in adults suspected of having tuberculosis showed poor sensitivity but good specificity.12 Test utility was greatly improved when the assay was used in combination with other tests to screen HIV-positive patients for tuberculosis before initiation of antiretroviral therapy; sensitivity increased in severely immunecompromised individuals and specificity remained high.13 These findings provided the rationale to assess the value of urinary lipoarabinomannan assays in children suspected of having tuberculosis because the collection of respiratory specimens is arduous in young children who are unable to expectorate, and occult haematogenous dissemination associated with primary infection might increase urinary lipoarabinomannan excretion. The present study included 535 children (median age 42·5 months) with suspected tuberculosis in whom urine and two induced sputum specimens were collected. Investigators collected urine specimens in urinary bags attached to the child’s perineum and frozen at –80°C. Both a lateral flow assay and ELISA assay (Clearview, Alere, Waltham, USA) were done on batched unfrozen specimens. Urinary lipoarabinomannan assays did not differentiate the 89 children with cultureconfirmed tuberculosis from those with possible tuberculosis (n=250) or no tuberculosis (n=196). Poor sensitivity (48·3% lateral flow assay; 2·2% Clearview) might have been expected in view of previous results in adults and the use of a reference standard based on respiratory specimens. However, the poor specificity (60·8% with lateral flow assay) and highly discrepant results reported with the various tests were unexpected. Less than 50% of specimens that tested positive (with a maximum 5 band intensity readout) on lateral flow assay tested positive with ELISA. These disappointing results might be indicative of the paucibacillary nature of paediatric tuberculosis with minimum lipoarabinomannan excretion in the urine. However, the fact that tests also did poorly in children with disseminated (miliary) tuberculosis makes this explanation unlikely. Other considerations include the possible effect of prolonged freezing of the urine specimens before testing, poor laboratory technique, or manufacturing errors. None of these explanations seem
Published Online April 8, 2014 http://dx.doi.org/10.1016/ S2214-109X(14)70207-4 See Articles page e278
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Comment
likely because the laboratory is highly experienced and followed the exact same protocol used in previous adult trials, and repeated assays that used kits with different batch numbers achieved similar results. The most plausible explanation for the poor specificity and poor intertest reliability might be related to the use of urine bags for specimen collection. Urine bags are attached to the perineum and are often left in situ for hours, with high rates of bacterial contamination and overgrowth. This method might account for the variable results achieved in the present study, whereas more reliable results were achieved in adult studies by use of clean-catch specimens. Although assessment of the potential effect of bacterial overgrowth on urine lipoarabinomannan tests might be of interest, no evidence exists that urine-based antigen or genetic14 tests have clinical application in children suspected of having tuberculosis. Further exploration of the utility of urinary lipoarabinomannan tests in children with possible disseminated (miliary) tuberculosis, in whom a rapid diagnosis is of paramount importance, seems warranted if urine collection can be optimised. In the meantime, the search for a reliable point-of-care test continues. Ben J Marais Clinical School, The Children’s Hospital at Westmead, Westmead, Sydney , New South Wales 2145, Australia
[email protected] I declare that I have no competing interests.
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Jenkins HE, Tolman AW, Yuen CM et.al. Incidence of multidrug-resistant tuberculosis disease in children: systematic review and global estimates. Lancet 2014; published online March 21. http://dx.doi.org/10.1016/S01406736(14)60195-1. Graham SM, Sismanidis C, Menzies HJ, et.al. Importance of tuberculosis control to address child survival. Lancet 2014; published online March 24. http://dx.doi.org/10.1016/S0140-6736(14)60420-7. Bates M, Mudenda V, Mwaba P, Zumla A. Curr Opin Pulm Med 2013; 19: 229–37. Marais B, Graham S, Maeurer M, Zumla A. Progress and challenges in childhood tuberculosis. Lancet Infect Dis 2013; 13: 287–89. Lawn SD, Mwaba P, Bates M, et al. Advances in diagnostic assays for tuberculosis: The Xpert MTB/RIF assay and future prospects for a point-of-care test. Lancet Infect Dis 2013; 13: 349–61. Perez-Velez CM, Marais BJ. Tuberculosis in children. N Engl J Med 2012: 367: 348–61. Mandalakas AM, Detjen AK, Hesseling AC, Benedetti A, Menzies D. Interferon-gamma release assays and childhood tuberculosis: systematic review and meta-analysis. Int J Tuberc Lung Dis 2011; 15: 1018–32. Nicol MP, Zar HJ. New specimens and laboratory diagnostics for childhood pulmonary TB: progress and prospects. Paediatr Respir Rev 2011; 12: 16–21. Nicol MP, Workman L, Isaacs W, et al. Accuracy of the Xpert MTB/RIF test for the diagnosis of pulmonary tuberculosis in children admitted to hospital in Cape Town, South Africa: a descriptive study. Lancet Infect Dis 2011; 11: 819–24. Rachow A, Clowes P, Saathoff E, et al. Increased and expedited case detection by Xpert MTB/RIF assay in childhood tuberculosis: a prospective cohort study. Clin Infect Dis 2012; 54: 1388–96. Nicol MP, Allen V, Workman L, et al. Urine lipoarabinomannan testing for diagnosis of pulmonary tuberculosis in children: a prospective study. Lancet Glob Health 2014; published online April 8. http://dx.doi. org/10.1016/S2214-109X(14)70195-0 . Lawn SD. Point-of-care detection of lipoarabinomannan (LAM) in urine for diagnosis of HIV-associated tuberculosis: a state of the art review. BMC Infect Dis 2012; 12: 103. Lawn SD, Kerkhoff AD, Vogt M, Wood R. Diagnostic accuracy of a low-cost, urine antigen, point-of-care screening assay for HIV-associated pulmonary tuberculosis before antiretroviral therapy: a descriptive study. Lancet Infect Dis 2012; 12: 201–09. Green C, Hugget JF, Talbot E, et al. Rapid diagnosis of tuberculosis through the detection of mycobacterial DNA in urine by nucleic acid amplification methods. Lancet Infect Dis 2009; 9: 505–11.
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