Table 1. Operational definitions of variables associated with the new tools for measuring retention in care.
| Variables | Definition | Numerator | Denominator |
| Retention | All patients who are not registered as deceased or lost to follow-up (LTFU) for any reason | Number of patients alive and on ART | Number of patients alive and on ART plus dead plus LTFU |
| Attrition | This is the opposite of retention, and refers to patients who discontinue care and treatment due to either death or LTFU. | Number of patients who either died or LTFU | Number of patients alive and on ART plus dead plus LTFU |
| Loss to follow-up | Patients who miss scheduled visits to the clinic within three months after the last visit. | Not applicable (NA) | NA |
| Transfer out | It refers to the official transfer of the patient to another clinic | NA | NA |
| Transfer in | It refers to the official transfer of the patient from another clinic | NA | NA |
| ART-age | The number of years that the patient was on ART | NA | NA |
| ART-age group | It refers to the age group that the patient belongs to based on the number of years the patient was taking ART during a specific the patient belongs to based on the calendar | NA | NA |
| ART-age cohort | It refers to the cohort that number of years that the patient was taking ART by the end of a specific calendar | NA | NA |
| Current retention | The retention rate during a specific “calendar” among patients who were on ART sometime during the “calendar”. The rate can be “ART-age group” specific or total. | Number of patients alive and on ART by the end of the calendar | Number of patients alive and on ART by the end of the calendar plus number of patients who died plus LTFU during the calendar |
| Cohort retention | The retention rate by the end of a specific “calendar” among a cohort of patients ever started on ART and followed longitudinally overtime. The rate can be “ART-age cohort” specific or total. The total “cohort retention” is similar to the cumulative retention rate. | Number of patients alive and on ART by the end of the calendar. Note: The numerators for current retention and cohort retention are similar. | Number of patients alive and on ART by the end of the calendar plus number of patients who died plus LTFU ever since patients were started on ART |
| Cumulative retention | The total “cohort retention” by the end of the calendar among patients ever started on ART | Number of patients alive and on ART by the end of the calendar | The total number of patients ever started on ART. |
| Calendar | The time during which or by the end of which the retention rate is measured (typically one year, e.g. 2008) | NA | NA |
| Probability of retention | The probability that a patient would be retained after “Y” number of years on ART. | NA | NA |
NA: Not applicable.