PPubMed30 May 2019Cited 33×

<i> Chlamydia trachomatis </i> samples testing falsely negative in the Aptima Combo 2 test in Finland, 2019

Kaisu Rantakokko-Jalava, Kati Hokynar, Niina Hieta, Anniina Keskitalo, Pia Jokela, Anna Muotiala, T. Sakari Jokiranta, Rutta Kuusela, Hannu Sarkkinen, Janne Aittoniemi, Tytti Vuorinen, Antti J Hakanen, Mirja Puolakkainen

Abstract

'Anniina Keskitalo' 'Pia Jokela' 'Anna Muotiala' 'T. Sakari Jokiranta' 'Rutta Kuusela' 'Hannu Sarkkinen' 'Janne Aittoniemi' 'Tytti Vuorinen' 'Antti J Hakanen' 'Mirja Puolakkainen'] Since February 2019, over 160 Chlamydia trachomatis (CT) cases testing negative or equivocal by Aptima Combo 2 (AC2) but positive by Aptima CT test run with Panther instruments occurred in Finland. The AC2 test targets chlamydial 23S rRNA while the CT test targets 16S rRNA. Sequencing of 10 strains revealed a nucleotide substitution in 23S rRNA. The significance of this for the failure of the AC2 test to detect the variant is not yet known.

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Finland’s Aptima Combo 2 test missed ~10‑15% of chlamydia cases due to a 23S rRNA variant, revealed by retesting with Aptima CT and sequencing.

What’s new

i.Identification of Aptima Combo 2 false‑negative/equivocal Chlamydia trachomatis cases in Finland.
ii.Implementation of systematic retesting with Aptima CT (ACT) based on RLU thresholds.
iii.Estimation of prevalence of missed cases across multiple laboratories (up to 15% of positives).
iv.Molecular characterization of the variant (genotype E with C→T change at 23S rRNA position 1515).
v.Recommendations for multi‑target testing and surveillance of diagnostic performance.

How they did it

The team defined cases as AC2‑negative or equivocal samples with RLU 20‑84 that were ACT‑positive, retested all such samples from February to April 2019, and sequenced the 23S rRNA of ten isolates. Prevalence was assessed by re‑analysing 943 AC2‑negative/equivocal samples from Turku and additional samples from other Finnish labs.

What they found

26 samples from 25 patients met the case definition (Feb‑Apr 2019).
Among 943 AC2‑negative/equivocal samples, 13 of 15 (87%) with RLU 20‑84 were ACT‑positive.
Overall, 15% of all chlamydia positives (15/98) were detected only by ACT.
In 2018, the proportion of AC2‑negative/equivocal but ACT‑positive samples was 0.4% of all tests and 6% of AC2‑positive reports.
Sequencing of 10 isolates showed all were genotype E with a single C→T substitution at 23S rRNA position 1515.

Caveats

✕The functional impact of the 23S rRNA C→T substitution on AC2 failure is not confirmed.
✕Only ten specimens were sequenced, limiting generalizability.
✕The study covered roughly 50 % of Finnish chlamydia screening, so national prevalence may differ.
✕Possible lower sensitivity of AC2 in female urine samples may contribute to false negatives.

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