Rapid point-of-care diagnosis is vital in cystic fibrosis (CF) management. The urine LAM test offers a promising approach for tuberculosis (TB) detection. However, frequent nontuberculous mycobacteria (NTM) infections in CF raise questions about test specificity. This study assesses urine LAM test performance in a CF cohort with high NTM rates.
A detailed study was conducted at the Copenhagen CF Center, known for its comprehensive CF care and high NTM prevalence but low TB incidence. Researchers rigorously screened CF patients for pulmonary NTM infection and tested urine samples from 198 patients using the Determine™ TB LAM Ag strip test (2013 version). Among these patients, 12% were found to have active pulmonary NTM infection, while no cases of active TB were present in this cohort. The TB-LAM test showed an overall positive rate of 2.5% when applying the recommended grade 2 cut-off for positivity, and this increased to 10.6% when a more sensitive grade 1 cut-off was used. Specifically, among patients with confirmed NTM infection, only 2 out of 23 (8.7%) had a positive LAM test result at the grade 2 cut-off, and 9 out of 23 (39.1%) at the grade 1 cut-off. The test demonstrated high specificity for NTM diagnosis, with 98.3% at the grade 2 cut-point and 93.1% at the grade 1 cut-point.
In conclusion, this research indicates low cross-reactivity of the urine LAM test with NTM infection in CF at the grade 2 cut-off. This supports its use for point-of-care TB diagnosis, particularly in TB-endemic areas. However, it is not suitable for point-of-care NTM diagnosis in CF patients.
