Epidemiologic studies of hemodialysis vascular access commonly compare incidence and prevalence access assessments and rates of vascular access-related hospitalization. We suggest the following definitions when referring to these parameters:
the proportion of patients who initiated an event or entered a study at a specified time point (e.g. the proportion of patients who initiated chronic hemodialysis therapy using an AVF, AVG or CVC in a given year). For example, we calculate the proportion of incident AVF use in dialysis unit X in 2011 as the number of patients using an AVF at the start of dialysis divided by all patients who started chronic HD from January 1, 2011 to December 31, 2011.
most commonly indicates “point prevalence” when referring to existing cases, particularly in vascular access studies. However, “period prevalence” has also been reported in the literature, thus both terms are defined.
- Point prevalence: percentage of patients using a specific vascular access at a certain point in time. For example, the point prevalent use of AVF on January 1, 2011 would be the number of patients using an AVF on January 1, 2011 divided by all patients using a vascular access on January 1, 2011 (multiplied by 100 for percentage).
- Period prevalence: percentage of patients using a specific vascular access over a certain period of time. For example, the period prevalent use of AVF in January, 2011 would be the number of patients using an AVF between January 1 and January 31, 2011, inclusive, divided by all patients using a vascular access during that same time period (multiplied by 100 for percentage).
Discussion: A key point is that when referring to “incident” and “prevalent”, it must clearly be stated what event(s) incident and prevalent are describing to limit erroneous assumptions. For example, when referring to incident AVF use, a patient must be dialyzing with an AVF at hemodialysis start. A non-functioning fistula in place at the initiation of dialysis should not be considered in the incident AVF calculation; in this instance, the incident access would be a catheter.
3. Hospitalization days/1000 access days
- Numerator is the total number of days of hospitalization for the study population
- Denominator is calculated by adding the days from access creation or start date of study period to permanent access failure (unsalvageable), end of study period, or access censored for patient death, transfer of dialysis unit or modality change (peritoneal dialysis or transplantation). The calculated rate of total number of hospitalizations/total number of access days is divided by 1000.
Discussion: A key point is that most publications do not differentiate between hospitalization for vascular access or non–vascular access related admissions. The indications for hospitalizations and their categorization is most accurately defined and determined a priori in prospective studies. Furthermore, most “access days” do not accurately count days an access is in use or in place. Often, the vascular access in place at a given time point (e.g. end of the month), is taken to be the access used for the entire time period. Thus, if a patient was using an AVF for the first 25 days of a month then required a catheter, only the catheter would be counted and may have 30 or 31 days erroneously attributed to its use for that month. The impact would be to increase the access days attributed to catheters and may minimize the severity of an outcome, such as catheter-related infectious hospitalization (e.g. the denominator would increase, reducing the rate/1000 catheter days). This highlights the importance of prospective tracking of vascular access creations/insertions and removals or censoring in a prospective electronic manner.
4. Complication Free Days (CFD)
This term has not been previously applied to the hemodialysis vascular access literature but is common in other literature, such as in the field of cardiology.
Complications include “serious vascular access events” and are comprised of any one of the following: access thrombosis, radiological or surgical intervention to facilitate or maintain patency, VA-related infections (see below for these definitions). Each event counts for 1 complication day. For example, if a graft had 2 angioplasties and 1 thrombolysis from the time of creation to 6 months (183 days), that graft would have 180 complication free days.
Often, these complications or serious vascular access events may be associated with extended problems, such as hospitalizations or need to use catheters with their accompanying risks. This can be described as CFD-extended. For example, in the above graft example, suppose the graft got infected after the last thrombolysis and required surgical resection of a graft component and 3 days of hospitalization. The “CFD-extended” would be 176 days (2 angioplasties + 1 thrombolyisis + 1 surgical revision + 3 hospital days).
In another 6 month example: in a patient who had a fistula created but failed to mature, requiring ligation of collateral vessels to faciliatate maturation and also requiring a central venous catheter for 6 weeks and developed CVC-related bacteremia, the fistula would have 181 CFD (1 day for ligation + 1 day for infection) and 139 CFD-extended (6 weeks or 42 weeks with a catheter, 1 ligation and 1 infection).
The ideal vascular access is one that provides adequate dialysis without complications; such an access would have extended patency with low maintenance and costs. While this “ideal” access might be achieved in a few, providing the most appropriate access for an individual can be a realistic expectation for all patients. The most appropriate access will be different for each patient and should consider important aspects of the patients clinical and life circumstances. One indicator for appropriate access is determining how many complications a patient may encounter with their access. Thus, an important measure is how many “complication free days” (CDF) a patient may have in a given timeframe using their access.
CFD and CFD-extended will allow a somewhat standardized comparison between vascular access types (given evaluation within similar time periods).
5. Access Abandonment
Determining the last day an access is used or is no longer available (similar to patient death but can be considered “access death”) is important for determining outcomes and denominators for event rates. Access abandonment is synonymous with final fistula or graft loss and access unsalvageable loss. It is defined as a fistula or graft that can no longer be used for 1 or 2 needle, prescribed dialysis as it may be unable to provide adequate flows and/or is deemed unsafe for the patient, and the associated problem cannot be corrected by any intervention, including medical, surgical, or radiological interventions or rest. See appendix
to confirm access abandonment.