Chlamydia 2018

Chlamydia 2018

Data from the MiBa

Since 2015, monitoring of chlamydia (oculo-genital infection caused by Chlamydia trachomatis) has been based on data from The Danish Microbiology Database (MiBa). The data comprise all positive and negative results from chlamydia tests from one department of clinical biochemistry and from all departments of clinical microbiology (DCMs) extracted from the MiBa. The data include the patients’ civil registration number, and disease episodes can therefore be delimited to the individual level. A new disease episode is defined as an interval of more than 42 days between two positive test episodes. Several negative test results within a single year are counted as a single negative testing episode. For patients with a valid civil registration number, information about the municipal code and thereby the area and region is collected from the Danish civil registration system. In the current annual report for 2018, numbers from 2017 are often provided to allow for comparison. Due to continuous updates made to data in the MiBa, each of these numbers differs marginally from the corresponding numbers published in EPI-NEWS 38/18.

Other data

The extent of chlamydia testing performed in private laboratories that market themselves via the internet remains unknown. Such data are not included in this annual report. 

When rectal chlamydia is established or when lymphogranuloma venereum (LGV; EPI-NEWS 20/2008) is suspected, some DCMs and STI clinics submit material to the SSI for relevant specialised testing.

Chlamydia incidence

In 2018, a total of 275,760 persons were tested for chlamydia (267,862 in 2017), and the number of detected cases was 33,415 (32,931 in 2017), Table 1. The annual incidence was 575 per 105 (570 per 105 in 2017 and 594 per 105 in 2016). In the 2004-2015 period, chlamydia monitoring was based on quarterly reports from the laboratories; and when calculating the positive rate, the denominator used was the number of analyses made. With data extracted from the MiBa as from 2010, the number of persons tested is used as the denominator. Therefore, the previously reported 9% positive rate from 2015 (previous method) is not directly comparable with the MiBa-based positive rates of 12.1% in 2018. The differences between the two methods is described in EPI-NEWS 34/16.

Sex and municipality were stated for 32,737 patients, and sex and age were stated for 32,931 patients. This explains the minor differences between the positive rates in some of the following tables.


Distribution of cases and incidence by sex and age group are presented in Table 2. Men accounted for 41% of the diagnosed cases. This proportion has increased steadily from 23% in 1994 to 38% in 2009-2012 and 40% in 2017. For both sexes, the majority of cases were observed among 15-29-year-olds: 80% among men and 88% among women, exactly as in 2017. The incidence was also significantly higher in this age group than in the group aged 30 years of age and above. Compared with 2017, the incidence for men had either increased or remained unchanged in all age groups, except for the age groups 40-44 years and 45-49 years, where the incidence had declined. Among women, the incidence declined in all age groups, barring 15-19 year-olds and all age groups from 35 years and older.


The incidences were higher in 2018 than in 2017 for nearly all combinations of sexes and one-year age groups. The highest incidence among men was found in 21-year-olds (20-year-olds in 2017). In women, the highest incidence was found among 19-year-olds, as in 2017.


Geographical distribution of chlamydia cases

Both overall and in the majority of areas, the incidence increased for both sexes in 2018 compared with 2017, Table 3.

The incidence declined among men in Copenhagen subs., on Bornholm and in West Jutland and North Jutland. Among women, the incidence decreased in Copenhagen subs., on Bornholm and in East Jutland.

Overall, the ratio between the male and female incidence remained unchanged, but decreased in Copenhagen subs. and West Jutland and increased in North Zealand and East Zealand.



Among the positive samples, 89% were submitted by general practitioners and 11% by hospitals (90% and 10%, respectively, in 2017). All cases were detected using nucleic acid amplification techniques. All laboratories reported positive urine sample findings.

Chlamydia was detected in urine samples in 13,296 cases, i.e. 40% of all cases (40% in 2017). Male samples constituted 86% of the positive urine samples (87% in 2017). Urine was used as sample material in 85% of the male chlamydia cases (89% in 2017 and 87% in 2016).

Chlamydia in children

Chlamydia was diagnosed in 145 children under 15 years (153 in 2017), including 80 aged 1-14 years (93 in 2017). In the 10-14-year age group, eight chlamydia cases were seen in boys and 69 in girls (9 and 74 cases, respectively, in 2017). The incidence per 105 among 10-14-year-olds was 42 for girls (45 in 2017) and 5 for boys (5 in 2017), Table 2. Probably sexually transmitted chlamydia (including findings in the throat and rectum) was detected in 69 girls, including 51 aged 14 years, 16 aged 13 years and two aged 12 years of age. Seven 14-year-old boys and one 13-year-old boy had chlamydia detected in the urine/urethra. In 62 children below one year of age, the diagnosis was based on samples from the eye. Of these infants, 54 were less than one month old, seven were one month old and one child was two months old. Two 6-week-old infants had chlamydia detected by pharyngeal swabs, and one of these also had an ocular swab.

The tested population

Age groups and sexes

As previously, the incidence of tested persons was considerably lower among men (2,792 per 105) than among women (6,683 per 105), Table 4, except for 0-year-olds and ≥ 50-year-olds.

Among both men and women, the incidence of tested people was slightly higher in 2018 than in 2017 for most age groups. Even so, the incidence decreased for girls aged 10-14 years of age. The positive rate remained practically unchanged, overall as well as in each age group.


Areas and sexes

Both totally and in most areas, the number of men and women tested per 105 increased from 2017 to 2018, but the increases recorded were very limited, Table 5.

For Denmark overall, the positive rate was slightly lower for men (17.0%) as well as for women (10.2%) than in 2017 (17.3% and 10.3%, respectively). For both men and women, a modest decline in the positive rate was observed in Copenhagen subs. and on Bornholm.


Testing of men for rectal chlamydia

The incidence of rectal testing of men, Table 6, increased to 153 per 100,000 in 2018 (136 per 100,000 in 2017). It increased in all areas except for West Jutland and East Jutland.

Rectal chlamydia was detected in 889 men (734 in 2017). The positive rate increased overall in Denmark and in all areas, except for East Jutland and North Jutland. The increase in West Jutland was larger than in other areas.


Lymphogranuloma venereum

Material was submitted to the SSI for LGV testing from 823 patients (516 men, 304 women and in three cases the sex was not stated). The number of ano-rectal samples was 710 (469 men, 240 women and in one case the sex was not stated). Thus, LGV testing was performed in 53% of men with rectal chlamydia.

LGV was detected in 63 men (13%) in the 23-63-year age group (in 54 ano-rectal samples, one urethral sample, one urine sample and in seven unspecified samples). Two women aged 21 and 24 years had rectal LGV detected.