3.3 Case Ascertainment

In active case ascertainment, surveillance personnel are hired and trained to abstract data from all data sources. Abstractors regularly visit or have electronic access to participating institutions, such as hospitals and clinics. Abstractors actively review multiple data sources such as log books, and medical, discharge and deaths records, to identify cases. For those fetuses or neonates identified in the log books as having a congenital anomaly, abstractors should request maternal and infant medical records to record relevant information onto a reporting form. Medical records need to contain relevant information in a format that can be readily identified and abstracted easily by the abstractors, who often have limited medical background.

• Advantages
  • This method usually improves case detection and case reporting, and improves data quality because more extensive clinical detail is collected.
• Disadvantages
  • This type of case ascertainment requires considerable surveillance programme resources and personnel.
  • The burden of work is placed on surveillance personnel.

In passive case ascertainment, congenital anomalies are reported directly to the surveillance programme. The information that is reported to the surveillance registry typically is not verified by direct abstraction of the medical record by surveillance personnel.

• Advantages
  • This type of case ascertainment is less expensive because fewer surveillance programme resources and personnel are required.
• Disadvantages
  • The burden of reporting falls on hospitals, clinics or other sources that may require time and effort from already busy staff.
  • Case detection and case reporting can be compromised because of the following:
    • Not all cases are reported, leading to an underestimate of the number of cases
    • Incomplete documentation, resulting in less detail on each case
    • Less timely reporting, leading to a delay in analyses and communication
    • Personnel may have varying levels of training and commitment, leading to inaccurate information
    • Stimulated/incentivized reporting may result in overestimation of certain congenital anomalies
    • Variation in reporting over time may generate spurious trends or hide real ones

Hybrid case ascertainment uses a combination of passive and active reporting systems. This method may use active case ascertainment to gather more detailed case information for specific congenital anomalies or to verify passive reporting for a percentage of all reported congenital anomalies, as a quality control measure. Hybrid ascertainment methods enable review of probable cases during follow-up and provide a definitive diagnosis, thereby reducing the number of births misclassified as cases.

• For example:
  • A surveillance programme can use active ascertainment of neural tube defects to gather more detailed case information in a timely manner, while also using passive ascertainment of the other congenital anomalies under surveillance.
  • A programme can use passive reporting with active follow-up verification of certain congenital anomalies to verify the accuracy of data submitted and gather more data.
  • A programme can conduct active case ascertainment from some sources, such as birthing hospitals, and accept passive reporting from other sources, such as cytogenetic laboratories.