Overview
- Standardized vocabularies, controlled vocabularies, terminologies and ontologies …
- This is a field of its own (health informatics)
- Let’s just call it “medical coding” for now.
ICD – International Classification of Diseases
- Maintained by the World Health Organization (WHO)
- Global standard for coding diseases and causes of death
- Used for:
- Clinical documentation
- Mortality statistics
- Epidemiological research
- Health system planning and monitoring
Historical Background
- First version: 1893 (International List of Causes of Death)
- WHO assumed responsibility in 1948 (ICD-6)
- Major revisions approximately every 10–20 years
- Each revision reflects:
- Advances in medical knowledge
- Changes in disease concepts
- Administrative and reporting needs
ICD has evolved from a mortality list to a comprehensive disease classification.
National Modifications
Several countries use national adaptations:
- ICD-10-CM (USA; Clinical Modification)
- ICD-10-CA (Canada)
- ICD-10-SE (Sweden)
What Does an ICD Code Represent?
An ICD code reflects:
- Clinical documentation
- Coding rules
- Administrative structure
- Local practice
Crosswalks Between Versions
When analyzing long time series:
- Mapping tables (“crosswalks”) are often used
- Mapping may be:
- One-to-one
- One-to-many
- Many-to-one
Crosswalks are rarely exact. Information loss or ambiguity is common.
Aggregation to broader diagnostic groups is often necessary.
ATC for drugs
- Anatomical Therapeutic Chemical (ATC) classification
- categorizing therapeutic drugs,
- introduced in the 1960s
- In 1980, the World Health Organization (WHO) recommended the ATC system as the “state of the art”
Procedure codes
- We use ICD for diagnoses and medical condition
- But how are patients with such diagnosis treated?
- What actions (in addition to the prescription of medicines) do we have?
- USA has a special version of ICD for this: ICD-10-PCS
- PCS = Procedure Coding System
NOMESCO
In Sweden, medical procedures are coded using the NOMESCO Classification of Surgical Procedures (NCSP).
- Developed by the Nordic Medico-Statistical Committee (NOMESCO)
- Used in Sweden, Denmark, Finland, Norway, and Iceland
- Primarily for on surgical procedures
KVÅ
- In Sweden implemented through KVÅ (Klassifikation av vårdåtgärder)
- Maintained nationally by Socialstyrelsen
- Includes the NOMESCO-NCSP codes for surgery
- Also includes additional codes for non-surgical treatments and activities
- Administration of chemotherapy (cytostatic treatment)
- Radiotherapy sessions
- Dialysis treatment (hemodialysis, peritoneal dialysis)
- Blood transfusion
- Vaccination
- Advanced wound care (non-surgical)
- Multidisciplinary team conference (MDT conference)
- …
SNOMED CT – What Is It?
SNOMED CT (Systematized Nomenclature of Medicine – Clinical Terms) is a large clinical terminology system.
- Maintained by SNOMED International
- Contains hundreds of thousands of clinical concepts
- Designed for structured documentation in electronic health records
Unlike ICD or ATC, SNOMED CT is primarily a terminology, not a statistical classification.
Terminology vs Classification
| ICD |
Classification |
Epidemiology and health statistics |
| ATC |
Classification |
Drug classification |
| KVÅ / NOMESCO |
Classification |
Medical procedures |
| SNOMED CT |
Terminology |
Detailed clinical documentation |
Classification systems simplify reality for statistics and reporting, while terminologies allow very detailed clinical descriptions.
Regular Expressions (Regex)
- A way to describe patterns in text
- Used to:
- Identify diagnosis codes (ICD-10)
- Identify drug codes (ATC)
- Clean register data
- Validate variables
EL8: Health care registers
Types of registers
National health registers
- mandated by law
- Sometimes with regional data collection and yearly updates
- Governed by state authority
Quality registers
- Volontary for health care providers but commonly adopted
- opt-out for individuals
- more than 100 in Sweden
- But similar data collections in other countries by other names
Swedish Medical Birth Register
- Established in 1973
- Covers pregnancies resulting in delivery in Sweden
- Includes live births and stillbirths from gestational week 22+0
- Contains information reported by maternal care, delivery care, and neonatal care
The Total population Register (TPR)
- maintained by Statistics Sweden (SCB)
- based on data from the Population registration
- structured for statistical analysis and research
- used as the sampling frame for surveys and register-based studies
Childhood
During childhood and school years, health information is collected through:
- Child health services (BVC)
- School health services
- Vaccination records
- Examinations by school nurses or physicians
- dentists etc
In contrast to many other stages of life in Sweden, these data are not collected in a national register.
Vaccination Register
- National register maintained by the Public Health Agency of Sweden (Folkhälsomyndigheten)
- Established in 2013
- Primarily covers vaccinations given within national vaccination programmes
Military conscription
The conscription register contains information from military conscription examination
- physical measurements (e.g. height, weight), cognitive ability tests, psychological assessments, health status and diagnoses, physical fitness
- covers most Swedish men born roughly 1951–1990
- Inactive 2010-2016
- Both men and women since 2017 (but only approximately 25 %)
- examinations typically performed at age 18–19
- data collected by the Swedish Armed Forces
Swedish Dental Health Register
- National register maintained by the National Board of Health and Welfare (Socialstyrelsen)
- Established in 2008
- Includes adults (≥20 years)
- based on reports submitted within the national dental care subsidy system
- includes both public and private dental care providers
Screening registers
- breast cancer (mammography)
- cervical cancer
- colorectal cancer
These programmes aim to detect disease at an early stage in otherwise healthy individuals.
Data from screening programmes are often stored in regional systems
National Patient Register (NPR)
- National register maintained by the National Board of Health and Welfare (Socialstyrelsen)
- Covers specialised health care in Sweden
- Data are collected by regions and healthcare providers and then reported to the national register
Coverage
- inpatient care since the 1960s
- nationwide coverage since 1987
- specialised outpatient care since 2001
Main variables include:
- diagnoses (ICD codes)
- procedures (KVÅ codes – Swedish classification of healthcare interventions)
- dates of admission and discharge
- hospital or clinic
- patient demographics
Primary care data
Primary care accounts for a large share of health care contacts in Sweden, but there is still no comprehensive national primary care register comparable to NPR.
- most primary care data are stored in regional electronic health record systems
- reporting practices differ between regions
- national coverage for research and statistics is therefore limited
Prescribed Drug Register (PDR)
- National register maintained by the National Board of Health and Welfare (Socialstyrelsen)
- Covers dispensed prescription drugs from Swedish pharmacies
- Established in 2005
- Nationwide coverage
PDR does not include
- drugs administered in hospitals
- over-the-counter drugs
- since 2009, many non-prescription drugs can also be sold outside pharmacies (e.g. supermarkets and petrol stations)
- most herbal medicines or dietary supplements
Dispension does not guarantee that the patient actually used the medication
Cancer Register
- National register maintained by the National Board of Health and Welfare (Socialstyrelsen)
- Established in 1958
- Covers all newly diagnosed malignant (and some benign) tumors in Sweden
- Based on “primary tumour”
Cause of Death Register
- National register maintained by the National Board of Health and Welfare (Socialstyrelsen)
- Established in 1952
- Covers all deaths among persons registered in Sweden
- Information is based on death certificates completed by physicians.
- Diagnoses are classified using ICD codes. Currently ICD-10 (WHO version, not ICD-10-SE)
LISA
Not a health care register but often used for background data.
- education level
- income and taxation
- employment status
- occupation and workplace
- social insurance benefits
- family situation
National Quality Registers
National quality registers collect detailed clinical information on specific diseases or treatments.
They are designed to:
- monitor and improve quality of care
- support clinical quality improvement
But they can also be used for research
Typical characteristics:
- focus on specific diagnoses, treatments, or procedures
- contain more detailed clinical data than national health data registers
- participation is generally voluntary for healthcare providers
There are currently around 100 national quality registers in Sweden.
The Swedish register ecosystem
Key characteristics:
- Nationwide coverage for many registers
- Longitudinal data spanning several decades
- Possibility of individual-level linkage across registers by the individual personal number
Limitations:
- some sectors lack national registers (e.g. primary care, school health services)
- coverage and data quality may vary across registers, health care providers (private vs public, secondary vs “tertary” care and over time)
Countries with similar register infrastructures
The Nordic countries have relatively similar systems based on:
- nationwide administrative registers
- universal healthcare systems
- personal identification numbers
These countries are therefore often used in comparative register-based research.
Examples from other countries
Health data systems in other countries are often organised differently:
- United Kingdom: NHS administrative datasets
- Netherlands: population registers linked with health insurance data
- United States: insurance claims databases and cohort studies
- Canada: provincial administrative health data
These systems can provide valuable data, but often lack:
- nationwide coverage
- consistent linkage across registers
- very long follow-up periods