"Lag Time vs. Third Next Available: Which Metric Really Reflects Patient Access?"

For years, Third Next Available Appointment (TNAA) was one of the most widely cited metrics for measuring access. But as health systems deepen their understanding of ambulatory supply and demand, many leaders are asking a simple question: Does TNAA actually reflect how long patients wait for care?

A recent discussion among Patient Access Collaborative members suggests that the answer may be more complicated than many leaders may assume.

A Metric with a Specific Origin

TNAA traces its roots to the early Advanced Access movement. The concept was introduced in primary care as a way to estimate how long a patient might wait for an appointment by counting the number of days until the third available slot on a provider’s schedule. The third slot was chosen because the first two openings often result from cancellations and therefore may not represent true availability.

The idea was simple and operationally practical. Over time, however, TNAA spread far beyond its original purpose. Today it is often used across specialties, service lines, and organizations, sometimes with widely different operational definitions.

That variation has led some access leaders to question whether TNAA provides a reliable signal of system performance.

The Snapshot Problem

Several members pointed out that TNAA functions largely as a moment-in-time measurement. Because it reflects the current state of the schedule, the number can shift dramatically with cancellations, template changes, or schedule releases.

In other words, TNAA can act more like a spot check than a comprehensive measure of access. It also raises a practical question. How many patients call and ask for the third next available appointment? In reality, patients care about something much simpler: when they can be seen.

Why Lag Time Is Gaining Ground

For that reason, many organizations in the discussion reported focusing more heavily on lag time, often measured as the number of days between when an appointment is requested and when it is scheduled.

Lag time offers several advantages.

First, it captures actual patient experience. Instead of estimating access based on open slots, it measures how long real patients wait for care.

Second, lag time can be aggregated across large volumes of appointments, making it easier to track trends and evaluate improvement efforts. Several leaders emphasized that median lag time can be particularly powerful because it reduces the influence of outliers.

Some organizations are also refining the metric further by capturing a requested date for care. In this model, lag represents the difference between when a patient or provider hoped the visit would occur and when the appointment was ultimately scheduled. That distinction adds an important layer of context to the measurement.

Measurement from Two Perspectives

Participants also highlighted an interesting analytical approach: measuring lag in two ways. One method looks at lag based on appointment creation date. This answers the question: How far into the future were patients calling yesterday scheduled? The other examines lag based on appointment date, which answers: How long ago were the appointments taking place today actually scheduled?

Together, these perspectives can reveal important patterns in demand, scheduling behavior, and access performance.

Not Either Or

Despite the growing preference for lag time, many access leaders still track both metrics. TNAA can provide a quick operational signal about schedule availability. Lag time, however, often offers a more complete picture of patient waiting time across the organization.

The key is understanding what each metric is actually measuring.

As the conversation highlighted, access improvement depends on measuring the right signals. When the metric reflects real patient experience, it becomes a far more powerful tool for identifying barriers, guiding operational change, and ultimately delivering care when patients need it most.