Insulin Dosage

Insulin dosing requires two primary calculations for tight glycemic control. Insulin-to-Carbohydrate Ratio (I:C or ICR): Represents grams of carbohydrate covered by 1 unit of rapid-acting insulin. Common ratios range 1:5 to 1:20, varying by insulin sensitivity. For example, 1:10 ratio means 1 unit insulin covers 10 grams carbohydrate. A meal with 60 grams carbs requires 60÷10 = 6 units. 1:15 ratio (more insulin-sensitive): Same 60-gram meal requires 60÷15 = 4 units. Starting ratio estimation: Use the 450 rule for rapid-acting insulin (Humalog, Novolog, Apidra) or 500 rule for Regular insulin: I:C ratio = 450 ÷ Total Daily Dose (TDD). For someone using 50 units total daily insulin: 450÷50 = 1:9 ratio. If using Regular insulin: 500÷50 = 1:10 ratio. Ratios often vary by meal due to diurnal insulin sensitivity variation—breakfast typically requires more insulin (1:8-1:10), lunch/dinner may need less (1:12-1:15). Correction Factor (CF) or Insulin Sensitivity Factor (ISF): mg/dL decrease in blood glucose per 1 unit insulin. Common values: 20-100 mg/dL per unit. More insulin-resistant individuals have lower CF (20-40 mg/dL per unit), insulin-sensitive have higher CF (50-100 mg/dL per unit). Calculation using 1800 rule (rapid-acting) or 1500 rule (Regular): CF = 1800 ÷ TDD. For TDD 50 units: CF = 1800÷50 = 36 mg/dL per unit. If blood glucose is 220 mg/dL and target is 100 mg/dL, correction needed = (220-100)÷36 = 3.3 units (round to 3 units). Total bolus dose = carbohydrate dose + correction dose. Example: Pre-meal glucose 180 mg/dL, target 100 mg/dL, meal has 75 grams carbs. I:C ratio 1:12, CF 40 mg/dL/unit. Carb dose = 75÷12 = 6.25 units. Correction dose = (180-100)÷40 = 2 units. Total bolus = 6.25 + 2 = 8.25 units (round to 8 units).

Insulin therapy mimics physiological insulin secretion using basal (background) and bolus (mealtime) insulin components. Rapid-Acting Analogs: Insulin lispro (Humalog), aspart (Novolog), glulisine (Apidra): Onset 10-15 minutes, peak 60-90 minutes, duration 3-5 hours. Taken immediately before or with meals for carbohydrate coverage and corrections. Clear appearance. Short-Acting (Regular): Humulin R, Novolin R: Onset 30 minutes, peak 2-4 hours, duration 5-8 hours. Taken 30 minutes before meals. Less commonly used due to delayed action. Clear appearance. Intermediate-Acting (NPH): Humulin N, Novolin N: Onset 1-2 hours, peak 4-8 hours, duration 12-18 hours. Cloudy appearance (requires mixing). Provides basal coverage, typically dosed twice daily. Unpredictable peaks increase hypoglycemia risk. Long-Acting Analogs: Insulin glargine (Lantus, Basaglar, Toujeo): Onset 1-2 hours, no pronounced peak, duration 20-24 hours. Dosed once daily (same time). Insulin detemir (Levemir): Onset 1-2 hours, minimal peak, duration 18-24 hours. Often requires twice-daily dosing. Ultra-Long-Acting: Insulin degludec (Tresiba): Onset 30-90 minutes, no peak, duration >42 hours. Most stable basal insulin. Allows flexible timing. Premixed Insulins: Combinations like 70/30 (70% NPH, 30% Regular) or 75/25 (75% intermediate, 25% rapid). Fixed ratios limit flexibility. Basal-Bolus Regimen (physiologic): Basal insulin (long-acting): Typically 40-50% of TDD, provides 24-hour background coverage independent of meals. Dosed once or twice daily. For example, TDD 50 units → 25 units basal (glargine at bedtime). Bolus insulin (rapid-acting): Remaining 50-60% divided across meals based on carbohydrate content. Same person: 25 units divided as breakfast 8 units, lunch 8 units, dinner 9 units. Adjustments based on meal size using I:C ratios. Twice-Daily NPH Regimen: 2/3 TDD in morning (2/3 NPH, 1/3 Regular), 1/3 TDD in evening (1/2 NPH at bedtime, 1/2 Regular at dinner). Less physiologic, higher hypoglycemia risk.

Type 1 Diabetes (insulin-dependent, no endogenous production): Weight-based calculation: TDD = 0.4-1.0 units/kg body weight. Wide range reflects disease duration and residual beta-cell function. Newly diagnosed (honeymoon period): 0.4-0.6 units/kg. For 70 kg person: 28-42 units daily. Residual insulin production reduces needs. Established diabetes (>5 years): 0.6-0.8 units/kg. For 70 kg: 42-56 units daily. Long-standing or brittle diabetes: 0.8-1.0+ units/kg. For 70 kg: 56-70+ units daily. Increased insulin resistance over time. Children and adolescents: Prepubertal: 0.6-0.7 units/kg. Pubertal (growth hormone surge): 1.0-1.5 units/kg due to insulin resistance. May require 70-100 units daily even at lower body weights. Type 2 Diabetes (insulin-resistant, variable endogenous production): Initial dosing (insulin-naive): Start conservatively at 0.1-0.2 units/kg for basal insulin (long-acting) to minimize hypoglycemia risk. For 90 kg person: 9-18 units glargine once daily. Titrate by 2 units every 3 days based on fasting glucose. Established insulin therapy: 0.5-1.5 units/kg depending on insulin resistance severity, obesity, and medication regimen. For 90 kg person: 45-135 units daily. Basal-bolus in Type 2: Higher TDD than Type 1 due to insulin resistance—often 1.0-2.0 units/kg (90-180 units for 90 kg person). Factors increasing insulin requirements (+20-100%): Obesity: Insulin resistance increases 10-20% per 10 kg excess weight. Illness/infection/stress: Increases needs 25-50% acutely. Steroids (prednisone): Increase requirements 50-100%. Dose 20 mg prednisone daily may double insulin needs. Pregnancy: Requirements increase 50-100% in 3rd trimester due to placental hormones. Factors decreasing insulin requirements (-20-50%): Exercise: Acute activity decreases needs 20-30% for 24-48 hours. Weight loss: Each 10 kg loss reduces requirements 10-20%. Improved diet/carb restriction: Lower carb intake reduces bolus needs proportionally. Gastroparesis: Delayed stomach emptying requires delayed or split boluses.

Sliding Scale Insulin (SSI)—Traditional/Reactive Approach: Fixed insulin doses based solely on current blood glucose level, ignoring carbohydrate intake or future insulin needs. Example sliding scale: Glucose <150 mg/dL: 0 units; 151-200: 2 units; 201-250: 4 units; 251-300: 6 units; >300: 8 units + call physician. Major limitations: (1) Reactive, not proactive: Treats high glucose after it occurs rather than preventing it. (2) Ignores meal carbohydrates: Same insulin dose given whether eating 30 or 90 grams carbs. (3) Promotes glucose variability: Large swings between hyper- and hypoglycemia. (4) "Chasing" glucose: Correcting high glucose from previous meal while adding new meal creates insulin stacking and delayed hypoglycemia. (5) Poor A1C outcomes: Studies show A1C averages 0.5-1.0% higher than physiologic dosing. Only appropriate use: Hospitalized patients with variable intake and insulin-resistant conditions (steroids, critical illness) where carb counting impractical. Carbohydrate Counting with Correction—Physiologic/Proactive Approach: Calculates insulin based on planned carbohydrate intake plus correction for current glucose, preventing hyperglycemia rather than treating it. Calculation: Total bolus = (Carbs ÷ I:C ratio) + [(Current BG - Target BG) ÷ Correction Factor]. Example scenario: Pre-lunch glucose 180 mg/dL, target 120 mg/dL, eating 60 grams carbs. I:C ratio 1:10, CF 40 mg/dL/unit. Carb dose = 60÷10 = 6 units. Correction = (180-120)÷40 = 1.5 units. Total = 7.5 units (round to 7-8 units). Advantages: (1) Proactive: Prevents post-meal hyperglycemia. (2) Flexibility: Eat variable meals without glucose spikes. (3) Better A1C: Typically 0.8-1.2% lower than sliding scale. (4) Reduced hypoglycemia: Appropriate insulin for actual food intake. (5) Mimics physiology: Matches insulin to glucose load. Requirements: Understanding carbohydrate content (reading labels, estimating portions), accurate I:C ratio and CF determination through testing and adjustment. Advanced strategies: Food Effect Factor: Adjust for protein and fat in mixed meals. High-fat meals (pizza, fried foods) may require 20-30% more insulin or extended/dual-wave bolus on pumps. Activity adjustment: Reduce bolus 10-50% before exercise to prevent hypoglycemia. Alcohol consideration: Alcohol inhibits gluconeogenesis, increasing hypoglycemia risk 8-12 hours post-consumption—reduce evening basal by 20-30%.

Insulin Stacking (Insulin On Board/IOB): Occurs when additional insulin is given before previous doses are fully metabolized, leading to additive effects and severe hypoglycemia. Rapid-acting insulin remains active 3-5 hours, but many patients re-dose within 2-3 hours, creating dangerous overlap. Active Insulin Time (Duration of Insulin Action/DIA): Time from injection until insulin effect is negligible. Rapid-acting analogs: 4-5 hours for most people (range 3-6 hours based on injection site, dose, individual metabolism). Regular insulin: 6-8 hours. Modern insulin pumps and calculators track IOB using DIA setting—typical pump default is 4 hours, but individual variation requires personalization. Calculating remaining active insulin: Uses linear decay model (though actual insulin action is curved). Example: DIA set to 4 hours. Gave 8 units bolus 2 hours ago. Remaining IOB = 8 × (time remaining ÷ DIA) = 8 × (2 hours ÷ 4 hours) = 4 units still active. If current glucose is 200 mg/dL, target 120 mg/dL, CF 40 mg/dL/unit, correction calculation: Needed correction = (200-120) ÷ 40 = 2 units. But 4 units IOB means no additional correction needed—active insulin will lower glucose approximately 4 × 40 = 160 mg/dL, from 200 to 40 mg/dL without more insulin. Taking additional 2 units would cause severe hypoglycemia (predicted low of -80 mg/dL theoretically). Preventing insulin stacking: (1) Wait minimum 3-4 hours between correction doses unless eating additional carbohydrates. Exception: severe hyperglycemia >300 mg/dL may warrant earlier re-dosing with caution. (2) Use IOB-aware insulin calculators (pump bolus calculators, smartphone apps, CGM systems) that automatically subtract IOB from recommended dose. (3) Document all insulin doses with timing in logbook or app to track active insulin manually if needed. (4) Avoid rage bolusing—giving large doses out of frustration with persistent highs. Insulin works slowly; patience prevents overcorrection. Special considerations: Exercise effect: Physical activity accelerates insulin absorption 20-50%, especially from exercising limbs, effectively reducing DIA to 2-3 hours and increasing stacking risk. Sick days/stress: Insulin resistance may prolong apparent DIA because same insulin has less effect, appearing to "last longer." Gastroparesis: Delayed stomach emptying means carbs absorbed over 4-6 hours, requiring extended boluses on pumps or split dosing with pens. Single large bolus creates mismatch with early insulin action, causing initial hypoglycemia then late hyperglycemia.

CGM systems (Dexcom, Freestyle Libre, Guardian) provide real-time glucose values every 1-5 minutes with trend arrows indicating rate and direction of change, enabling proactive insulin adjustments beyond static blood glucose values. CGM trend arrows and insulin adjustment: → (Flat/Stable): Glucose changing <1 mg/dL/min. Use standard I:C and CF without modification. If at target (80-180 mg/dL) and flat, no action needed even if technically outside ideal range—avoid overtreatment. ↗ (Slowly Rising): Glucose rising 1-2 mg/dL/min (60-120 mg/dL/hour). Indicates underestimated carbs, insufficient bolus, or impending stress hyperglycemia. Action: Add 10-20% to correction dose if high, or give small correction (1-2 units) even if currently in range to prevent continued rise. ↑ (Rapidly Rising): Glucose rising 2-3 mg/dL/min (120-180 mg/dL/hour). Indicates significant insulin deficit—missed bolus, pump site failure, illness. Action: Add 20-40% to correction dose. Check ketones if Type 1 and >250 mg/dL. Consider pump site change if no obvious cause. ↘ (Slowly Falling): Glucose falling 1-2 mg/dL/min. May indicate appropriate post-meal response or slightly excessive insulin. Action: If in range (100-180) and slowly falling, no action. If heading toward 80 mg/dL, consider 5-10 gram preventive carbs. ↓ (Rapidly Falling): Glucose falling 2-3 mg/dL/min. High hypoglycemia risk within 15-30 minutes. Action: Treat proactively even if currently in range. At 120 mg/dL falling rapidly, take 10-15 grams fast carbs immediately—will likely reach 70-80 mg/dL within 20 minutes without treatment. Delay any planned boluses. Pattern-based basal adjustments: Overnight (midnight-6am): Should be stable ±30 mg/dL. Consistent rise >30 mg/dL indicates insufficient basal; increase long-acting by 1-2 units or adjust pump basal rates. Consistent fall or lows indicate excessive basal; decrease. Fasting (pre-breakfast): Target 80-130 mg/dL. Consistent fasting values >150 or <70 warrant basal adjustment ±10%. Between meals: Should return to pre-meal baseline within 4-5 hours. Failure to return suggests incorrect I:C ratio—persistent elevation means more insulin needed (lower ratio, e.g., 1:12 → 1:10). Hypoglycemia before next meal indicates too much insulin (raise ratio, e.g., 1:10 → 1:12). Time-in-range (TIR) goals: Target >70% time 70-180 mg/dL, <25% time >180 mg/dL, <4% time <70 mg/dL, <1% time <54 mg/dL. If TIR <70% with high variability, focus on preventing lows first (reduce insulin), then address highs. Low variability is safer than tight control with frequent hypoglycemia. Glucose Management Indicator (GMI): CGM-calculated estimated A1C. GMI 7.0% corresponds to average glucose 154 mg/dL. Each 1% GMI change ≈ 30 mg/dL average glucose change.