BACKGROUND: Primary care doctors put diagnostic codes on all reimbursement cards. The objective of this study was to map out the use of non-specific diagnostic codes that can undermine the validity of statistics and disease surveillance. MATERIAL AND METHOD: The material consists of data from all electronic reimbursement cards from out-of-hours services in the period 2008-2019. We registered consultations and telephone contacts and the proportion of these that were supplied with diagnostic codes for respiratory infections and three non-specific diagnostic codes. RESULTS: The number of consultations per year increased from 1 402 452 in 2008 to 1 417 395 in 2019, a relative increase of 1 %. The number of telephone contacts per year increased from 286 515 in 2008 to 684 773 in 2019, a relative increase of 139 %. Out-of-hours contacts coded with non-specific diagnoses increased nearly thirteenfold, from 40 280 to 514 715. The use of non-specific diagnoses increased by a factor of 19 for telephone contacts and 2.7 for consultations. The total number of out-of-hours contacts for respiratory infections decreased from 240 037 to 176 909 (a 26 % reduction). INTERPRETATION: There is a strong tendency for general, non-specific diagnostic codes to replace specific diagnoses of disease on reimbursement cards from out-of-hours services. This undermines the evidence base for statistics and research based on reported ICPC-2 diagnoses, and this is especially of concern when these diagnoses are to be used for monitoring of the COVID-19 pandemic.