Blood glucose self-monitoring (SMBG) is a regular practice for people managing diabetes. Tracking these numbers provides an immediate assessment of how the body handles glucose from food, medication, or physical activity. This daily data collection is a direct and actionable way to understand the body’s metabolic response. The insights gained from SMBG allow for real-time adjustments to diet, exercise, and insulin or other diabetes medication dosages. Consistent monitoring is fundamental to the long-term management of blood sugar levels.
Understanding AC and HS Testing Times
The abbreviations AC and HS are medical notations specifying the exact times for blood glucose measurement. AC stands for Ante Cibum, which is Latin for “before meals.” This measurement is typically taken right before consuming breakfast, lunch, and dinner, giving a baseline reading prior to the digestive influx of carbohydrates.
The reading taken before breakfast is often referred to as a fasting blood glucose check, reflecting the glucose level after the overnight period without food. HS stands for Hora Somni, which translates to “at bedtime.” Checking blood glucose at this time is done immediately before going to sleep.
The HS measurement determines the safety of the overnight glucose level, a period when low blood sugar is difficult to detect or treat. Testing at these specific times captures different physiological phases. AC readings reflect basal insulin effectiveness, while HS readings indicate the starting point for overnight metabolic control.
Recommended Blood Glucose Target Ranges
Healthcare professionals establish individualized blood glucose goals, though standard guidelines exist for non-pregnant adults with diabetes. The accepted target range for the AC (pre-meal) reading is 80 to 130 milligrams per deciliter (mg/dL). This range serves as the desired metabolic baseline before the body is challenged by food intake.
The target range for the HS (bedtime) reading is often slightly higher than the pre-meal goal to mitigate the risk of nocturnal hypoglycemia. A common recommended range for HS is 90 to 150 mg/dL for adults. These goals are adjusted based on individual factors, including the type and duration of diabetes, age, the presence of other health conditions, and the risk of severe low blood sugar events.
What Deviations from Target Readings Signify
Readings that fall outside the target ranges provide important information about underlying physiological processes requiring adjustment.
High AC Readings (Fasting)
A consistently high AC reading, particularly the fasting morning number, may point to phenomena occurring overnight. One common cause is the Dawn Phenomenon, a natural early-morning surge of hormones (like cortisol and growth hormone) that triggers the liver to release stored glucose. Since the body with diabetes cannot produce enough insulin to counteract this release, the AC glucose level becomes elevated upon waking.
A high AC reading could also signal the Somogyi effect, which is a rebound from an undetected low blood sugar event during the night. This rebound occurs when an overnight drop in glucose triggers a release of counter-regulatory hormones, rapidly raising blood glucose to an excessively high level by morning.
To differentiate between the Dawn Phenomenon and the Somogyi effect, an additional blood glucose check around 3:00 a.m. for several nights is often needed. A low 3:00 a.m. reading followed by a high AC reading suggests the Somogyi effect, while a normal or high 3:00 a.m. reading suggests the Dawn Phenomenon.
High HS Readings (Bedtime)
High HS readings (above target) often suggest that the evening meal medication was insufficient or that a late-night, carbohydrate-heavy snack was consumed too close to sleep. This necessitates a review of the mealtime insulin dose or the timing of the evening meal.
Low HS Readings (Bedtime)
A low HS reading carries the risk of nocturnal hypoglycemia (low blood sugar during sleep). This can signal that the basal (background) insulin dose is too high, or that physical activity earlier in the day caused an unexpected drop in glucose. Adjustments may include reducing the evening medication dose or adding a small, balanced bedtime snack containing protein and complex carbohydrates to stabilize the glucose level through the night. Consistent tracking of both AC and HS levels allows a healthcare provider to pinpoint the time and cause of the fluctuation, leading to more precise adjustments in the diabetes management plan.