Why twelve doses a year is the denominator
Twelve monthly doses of heartworm preventive each year are the standard for dogs and cats in most of the United States. This expectation comes from label directions for most FDA-approved products, as well as guidance from national veterinary organizations. The risks of missing even one month are not just theoretical. Heartworm larvae can develop during any unprotected window, and it only takes a single lapse for infection to begin.
The arithmetic is simple: each patient that is due for year-round prevention should receive twelve doses in a calendar year. This is the denominator for measuring compliance. If a patient is under care for less than a full year, perhaps adopted mid-year, or transferred to your clinic in October, adjust the denominator accordingly. For most adult patients seen in wellness care, though, twelve is the number.
Annual antigen testing practices often assume this twelve-month protection as well. When a lapse occurs, the reliability of negative test results drops, and retesting or repeat testing schedules may need to adjust. This further reinforces why a full year of dosing matters, both for patient safety and for clinic protocols.
Keep reading: The Prescription Label Checklist for Every Vial You Dispense
Pulling doses dispensed per patient out of your practice software
Every major practice management system records prescription sales to individual patients. Pulling a report of heartworm preventive dispensed in the last twelve months is usually possible, though the process varies by platform. Most systems can filter by product type or drug code, and can group results by patient or by client.
The key is to focus on doses, not just transactions. A client who buys a six-pack twice gets credit for twelve doses. A client who buys a twelve-month supply gets the same. Some software offers a direct "quantity dispensed" field in the report, while others require exporting transaction records and adding up the totals.
Be mindful of refills dispensed through outside pharmacies or online portals if your practice software does not track those. Some clinics reconcile this data by asking clients to report outside purchases at wellness exams, though this is imperfect. For the most accurate picture, the data set should reflect all doses dispensed, regardless of source, though most clinics find it simplest to start with what they directly supplied.
Building a compliance rate that excludes patients you no longer serve
Not every patient listed in your database is still under your care. Including inactive patients skews your compliance numbers downward, making your practice look less effective than it is. Before running your heartworm compliance calculations, define your "active patient" group.
The most common approach is to count only patients seen for any service in the past eighteen months. Some clinics use twelve or twenty-four months, depending on appointment frequency. Patients marked as deceased or transferred out should be excluded. Software filters can automate some of this, but a manual check is often needed for edge cases.
Once you have your active patient pool, match each patient to the number of heartworm doses dispensed in the last twelve months. Divide the number of patients who received at least twelve doses by the total number of active patients. This gives you an annual compliance rate that reflects your real patient base, not a bloated list of inactive records.
Keep reading: Look Alike Names, Weight Bands, and Other Refill Errors to Catch
The gap month problem and how antigen test timing interacts with it
Even practices that achieve high annual compliance rates can face the "gap month" problem. This occurs when clients buy doses late or skip a month, creating a window when the pet is unprotected. The issue is not just about totals, but about spacing.
For example, a patient may receive all twelve doses in a year, but with a two-month gap in spring. The parasite exposure in those months still poses a risk. Practice management software typically cannot track exactly when each dose was administered, only when it was sold or dispensed. Unless your team logs "given" dates in the medical record, the timing of missed or late doses can be hard to reconstruct.
Antigen test timing compounds this challenge. Heartworm tests detect adult female antigens, which may not appear until six months after infection. If a pet misses a dose in March and tests negative in June, the test may not reveal a new infection caught during the gap. This lag means clinics may miss early infections and must consider both dose timing and test timing when counseling clients.
Some practices address this by flagging patients who buy less than twelve doses or who purchase in irregular patterns, and recommending a repeat test at six months post-lapse. This can be built into reminder protocols to catch patients at risk from the gap month effect.
Turning missed doses into testing risk and into lost pharmacy revenue
Missed heartworm doses do more than create medical risk, they also signal lost revenue opportunities for the clinic. Each missed dose is a preventive not dispensed, and the downstream cost of a heartworm infection far exceeds the price of a single dose. For patients who test positive, treatment is both complex and costly, but often reimbursed at lower rates than prevention.
From a compliance audit, tally the number of active patients who received fewer than twelve doses. Multiply the shortfall in doses by the unit sale price. This gives a direct estimate of lost preventive revenue, before considering the potential cost of additional testing or treatment.
The risk side is as important. Patients with missed months require different testing advice. After a lapse, the standard annual antigen test may not be enough to rule out infection. Consider whether your protocol includes retesting six months after the last missed dose, especially in areas with year-round transmission. This not only protects patients but can also justify additional follow-up visits and counseling.
Tracking this information helps prioritize patient outreach. Owners who missed a refill may benefit from targeted reminders, not only for the next due dose but also for the right time to retest. This closes the loop between compliance gaps, medical risk, and practice revenue.
See how WhiskerRefill handles this for veterinary medicine
Setting a reminder cadence from the numbers you just built
Once you have your compliance data, the next step is to design a reminder system that targets the most common gaps. If most clients fall short by one or two doses a year, focus reminders on those renewal months. If you see seasonal gaps, such as low sales in winter, tailor communications to explain the need for year-round protection.
There are several approaches to reminder cadence. The traditional model is a monthly refill reminder, sent before the next dose is due. Some practices prefer a quarterly or semi-annual approach, bundling reminders with wellness visit scheduling or other preventive care messages. A hybrid model sends more frequent reminders to clients with known compliance lapses, while steady clients receive a standard cadence.
Automated tools can help by matching refill reminders to actual purchase history, flagging owners who are due for a refill before they run out. Clinics without automation may rely on manual tracking, but this is labor-intensive and prone to error when the patient pool is large.
When setting cadence, consider the client's communication preference. Some owners respond to texts, others to email or phone. The most effective reminder is the one that reaches the owner in a way they will see and act on. Testing different approaches and tracking response rates over time helps refine the process.
Running the same math a quarter later to see if anything moved
After implementing a new reminder system or making changes to your compliance protocols, it's essential to measure the effect. Repeat your compliance calculation every quarter. Pull the same report of doses dispensed, match to your active patient list, and recalculate the rate of patients with twelve or more doses in the prior twelve months.
Look for changes not just in the overall compliance rate, but in the distribution of shortfalls. Are more patients reaching twelve doses? Has the average shortfall dropped from three months to one? Have seasonal gaps narrowed? These details tell you whether your interventions are working and where to fine-tune.
Share results with your team. When staff see a measurable improvement in compliance, they are more likely to stay engaged with reminder calls and follow-up tasks. If numbers are stagnant, review the process: Are reminders sent on time? Are clients receiving them in their preferred format? Are pharmacy staff and veterinarians reinforcing the message during visits?
For clinics looking to streamline, digital tools that track refill due dates, send owner reminders, and manage in-clinic refill requests can automate the loop between compliance measurement and intervention, freeing up staff time and making it easier to spot trends quarter by quarter.
