Clinical Ranges
| Population | normal | prediabetes | diabetes | diabetes diagnostic | preprandial target | postprandial target | fasting target | one hour postprandial | two hour postprandial |
|---|---|---|---|---|---|---|---|---|---|
| Non-diabetic adults (fasting) | 70-99 mg/dL (3.9-5.5 mmol/L) | 100-125 mg/dL (5.6-6.9 mmol/L) | >=126 mg/dL (>=7.0 mmol/L) | — | — | — | — | — | — |
| Non-diabetic adults (2-hour postprandial) | <140 mg/dL (<7.8 mmol/L) | 140-199 mg/dL (7.8-11.0 mmol/L) | >=200 mg/dL (>=11.1 mmol/L) | — | — | — | — | — | — |
| Random glucose (any time) | — | — | — | >=200 mg/dL with symptoms | — | — | — | — | — |
| Adults with diabetes (ADA targets) | — | — | — | — | 80-130 mg/dL (4.4-7.2 mmol/L) | <180 mg/dL (<10.0 mmol/L) | — | — | — |
| Pregnant women (gestational diabetes) | — | — | — | — | — | — | <95 mg/dL (<5.3 mmol/L) | <140 mg/dL (<7.8 mmol/L) | <120 mg/dL (<6.7 mmol/L) |
Overview
Blood glucose (blood sugar) measures the concentration of glucose circulating in the blood. Glucose is the primary energy source for cells and its regulation is critical for metabolic health. Abnormal blood glucose levels are the defining characteristic of diabetes mellitus and are associated with numerous short-term and long-term health complications.
In HealthKit, blood glucose data typically comes from continuous glucose monitors (CGMs) or traditional fingerstick blood glucose meters. This data is essential for diabetes management, metabolic health optimization, and identifying patterns related to diet, exercise, stress, and sleep.
How It's Measured
Continuous Glucose Monitors (CGM)
CGMs use a small sensor inserted under the skin (usually on the arm or abdomen) to measure glucose in the interstitial fluid:
- Sensor: Tiny electrode coated with glucose oxidase enzyme
- Measurement: Interstitial fluid glucose (not blood glucose directly)
- Frequency: Every 1-5 minutes depending on device
- Duration: Sensors last 7-14 days typically
- Calibration: Some require fingerstick calibration; newer models are factory-calibrated
Key CGM Limitations:
- 5-20 minute lag behind actual blood glucose
- Less accurate during rapid glucose changes
- Accuracy decreases in hypoglycemic range
- MARD (Mean Absolute Relative Difference) typically 9-15%
- Affected by pressure on sensor site, temperature, and hydration
Fingerstick Blood Glucose Meters
Traditional meters measure capillary blood glucose from a finger prick:
- Measurement: Direct blood sample via test strip
- Accuracy: FDA requires 95% of readings within 15% of lab values (for >100 mg/dL)
- Advantages: Direct blood measurement, no lag time, lower cost per test
- Disadvantages: Point-in-time only, requires finger prick, no trend data
CGM vs Fingerstick: Key Differences
| Aspect | CGM | Fingerstick | |--------|-----|-------------| | What's measured | Interstitial fluid glucose | Capillary blood glucose | | Frequency | Continuous (every 1-5 min) | Point-in-time | | Lag time | 5-20 minutes behind blood | No lag | | Trend data | Yes | No | | Typical MARD | 9-15% | 5-12% | | Hypoglycemia accuracy | Reduced | Better |
Clinical Recommendation: CGM values can differ from fingerstick by up to 20%. For treatment decisions, especially hypoglycemia, fingerstick confirmation is recommended.
Health Significance
Blood glucose monitoring is critical for:
- Diabetes management: Adjusting insulin, medications, diet, and activity
- Hypoglycemia prevention: Detecting and treating low blood sugar
- Prediabetes identification: Early intervention opportunity
- Metabolic health optimization: Understanding glucose response to foods
- Gestational diabetes monitoring: Protecting maternal and fetal health
Chronic hyperglycemia leads to:
- Microvascular complications: retinopathy, nephropathy, neuropathy
- Macrovascular complications: cardiovascular disease, stroke
- Increased infection risk
- Impaired wound healing
Clinical Interpretation Guidelines
Fasting Blood Glucose (8+ hours without food)
| Range (mg/dL) | Range (mmol/L) | Interpretation | |---------------|----------------|----------------| | 70-99 | 3.9-5.5 | Normal | | 100-125 | 5.6-6.9 | Prediabetes (IFG) | | >=126 | >=7.0 | Diabetes (confirm with repeat) |
Post-Meal (2-hour postprandial)
| Range (mg/dL) | Range (mmol/L) | Interpretation | |---------------|----------------|----------------| | <140 | <7.8 | Normal | | 140-199 | 7.8-11.0 | Prediabetes (IGT) | | >=200 | >=11.1 | Diabetes |
Hypoglycemia Thresholds
| Level | Threshold | Symptoms | Action | |-------|-----------|----------|--------| | Alert | <70 mg/dL | May be asymptomatic | Treat with 15-20g fast carbs | | Level 2 | <54 mg/dL | Shakiness, sweating, confusion | Immediate treatment | | Level 3 | Severe | Altered consciousness, seizure | Glucagon, emergency services |
Elevated Blood Glucose May Indicate
- Diabetes mellitus (Type 1, Type 2, or gestational)
- Prediabetes (impaired fasting glucose or impaired glucose tolerance)
- Medication effects: Steroids, thiazides, atypical antipsychotics
- Stress response: Illness, trauma, surgery (stress hyperglycemia)
- Hormonal disorders: Cushing's syndrome, acromegaly, pheochromocytoma
- Pancreatic disease: Pancreatitis, pancreatic cancer
Low Blood Glucose May Indicate
- Insulin or sulfonylurea excess (most common in diabetics)
- Missed or delayed meals in patients on glucose-lowering therapy
- Excessive alcohol intake (inhibits gluconeogenesis)
- Adrenal insufficiency
- Insulinoma (rare insulin-secreting tumor)
- Reactive hypoglycemia (post-meal in non-diabetics)
Red Flags for Immediate Consultation
- Blood glucose <54 mg/dL (<3.0 mmol/L) with symptoms
- Blood glucose >300 mg/dL (>16.7 mmol/L) with symptoms
- Signs of diabetic ketoacidosis (DKA): high glucose + nausea/vomiting + fruity breath
- Signs of hyperosmolar hyperglycemic state (HHS): extreme hyperglycemia + dehydration + confusion
- Recurrent hypoglycemia unawareness
- Persistent hyperglycemia despite medication adherence
CGM-Specific Metrics
For patients using CGM, additional metrics provide insight beyond individual readings:
Time in Range (TIR)
- Target range: 70-180 mg/dL (or personalized)
- Goal for most adults with diabetes: >70% time in range
- Hypoglycemia: <4% time below 70 mg/dL, <1% below 54 mg/dL
Glucose Management Indicator (GMI)
- Estimated HbA1c based on CGM average glucose
- Formula: GMI (%) = 3.31 + 0.02392 x mean glucose (mg/dL)
Glycemic Variability
- Coefficient of Variation (CV): Target <36% indicates stable glucose
- High variability associated with hypoglycemia risk and complications
Caveats & Limitations
CGM Limitations
- Lag time: Interstitial glucose lags blood glucose by 5-20 minutes
- Rapid changes: Less accurate during fast glucose rises or falls
- Hypoglycemia range: Accuracy degrades below 70 mg/dL
- Sensor issues: Compression artifacts, early sensor failures
- Calibration: Some devices require fingerstick calibration
- Cost: Significant ongoing expense for sensors
- Not FDA-approved for diagnosis: CGM is for monitoring, not diagnostic testing
Fingerstick Limitations
- User technique: Improper sampling affects accuracy
- Strip storage: Expired or improperly stored strips give errors
- Interfering substances: High vitamin C, acetaminophen (some meters)
- Hematocrit: Extreme anemia or polycythemia affects readings
- No trend data: Single snapshot misses patterns
General Limitations
- Individual variation: Same glucose level has different implications per person
- Context matters: Timing relative to meals, exercise, stress affects interpretation
- Medication interactions: Some drugs affect glucose or meter accuracy
- Hemoglobin variants: Affect HbA1c accuracy in some populations
Additional Notes
Meal Timing Metadata
HealthKit supports meal-related metadata for blood glucose samples:
- HKMetadataKeyBloodGlucoseMealTime: Indicates preprandial (before meal) or postprandial (after meal) timing
- This context is essential for proper interpretation of glucose values
Carbohydrate Counting Integration
Blood glucose data is most actionable when paired with:
- Dietary carbohydrate intake (HKQuantityTypeIdentifierDietaryCarbohydrates)
- Insulin delivery records (HKQuantityTypeIdentifierInsulinDelivery)
- Activity data (exercise affects glucose utilization)
Pattern Recognition
Clinicians and patients should look for:
- Dawn phenomenon: Early morning glucose rise (3-8 AM)
- Post-meal spikes: Which foods cause highest glucose excursions
- Exercise effects: Both acute and delayed hypoglycemia risk
- Stress and illness patterns: Sick day management needs
- Nocturnal hypoglycemia: Overnight lows may go undetected
Special Populations
- Elderly: May have higher glucose targets to avoid hypoglycemia
- Pregnant: Tighter targets for gestational diabetes
- Children: Age-appropriate targets; hypoglycemia awareness varies
- Athletes: Exercise-induced glucose fluctuations are normal
Technology Trends
- Closed-loop systems: CGM integrated with insulin pumps for automated delivery
- Non-invasive monitoring: Under development but not yet clinically validated
- CGM for non-diabetics: Growing use for metabolic health optimization