Some Chlamydia infections have been missed by screening tests

Since February, 2019, there have been about one hundred sexually transmitted Chlamydia infections in Finland where the primary screening result for Chlamydia has been negative, but where subsequent testing has shown samples to be positive for Chlamydia. This amount is less than 0.5 per cent of all samples tested.

Cases have been found in the laboratories of the hospital districts of Helsinki and Uusimaa, Päijät-Häme, Satakunta and Southwest Finland and in some private laboratories (SYNLAB and United Medix laboratories). All incorrect primary laboratory results observed so far have been corrected.

Chlamydia bacteria

Chlamydia bacteria.

Only a very small number of the tested patients have received an erroneous result. Since April 1, 2019, the laboratories have used confirmatory tests to ensure reliable results. The laboratories will review the old results and, at the discretion of the laboratories, patients may be called for renewed testing. The laboratories will inform independently of any follow-up measures. 

The phenomenon was first discovered at the Laboratory of Clinical Microbiology, Turku University Hospital, and the laboratory started immediately investigations in collaboration with other users of the test, with the University of Helsinki and with the manufacturer of the test. At the same time, the laboratories proceeded to use confirmatory tests to guarantee the reliability of Chlamydia testing.  It is not yet known why a small percentage of Chlamydia infections are not identified by the primary screening test.   

There are many sources of error in laboratory testing: The sampling technique and the type of sample affect the reliability of the Chlamydia test. 

Background information on Chlamydia infections 

Chlamydia is common sexually transmitted disease (STD) Finland. In 2018, about 280,000 tests for Chlamydia were carried out.  The National Institute for Health and Welfare maintains a register covering the contagious diseases in Finland, and almost 15,000 cases of Chlamydia were registered last year.  

Most Chlamydia infections are asymptomatic or only mildly symptomatic. Females may feel a burning sensation when urinating, have increased vaginal discharge (leukorrhea), lower abdominal pain and intermenstrual bleeding. Males may have a burning sensation when urinating and a grey discharge from the urethra. 

If there are symptoms suggestive of a STD, if the partner has been diagnosed with or suspected of chlamydia infection, if the partner is diagnosed with any other STD, if the sex partner is recent or the number of partners high, there is cause to check for STD.  Testing and treatment of Chlamydia in public health care centres and in hospital outpatient clinics for STD are free of charge for the patient. 

Chlamydia and other STDs can be prevented by using condoms and oral sex protection.


For more information on Chlamydia infections, please visit:

Terveyskylä (In Finnish)

THL (In Finnish)


Photo: iStock

Modified : 17/12/2019 09:45
Created : 24/04/2019 09:15