Insulin
TDD → correction → quantity in one flow
Step 1 — Patient profile
Step 2 — TDD range and your choice
Enter weight and type to compute TDD.
Step 3 — Correction factors
Pick a dose in Step 2.
Step 4 — Basal product and quantity
Step 5 — Bolus product and quantity
Weight-based total daily dose + basal/bolus split
Patient Weight
Diabetes Type
ISF and ICR (rule-of-1800 / rule-of-500)
Total Daily Dose
Insulin Type
What these two numbers mean
Both are derived from the same input — the patient's total daily dose (TDD) of insulin, basal plus bolus — and both describe what one unit of rapid-acting insulin does.
- ISF / correction factor — how far 1 unit is expected to lower blood glucose, in mg/dL. Answers: "BG is 250, how much do I give to bring it down?"
- ICR — insulin-to-carb ratio — how many grams of carbohydrate 1 unit is expected to cover. Answers: "she's about to eat 60 g, how much does that meal need?"
A mealtime dose in a patient who is already high is the sum of the two: carb coverage + correction.
Where the formulas come from
| Insulin | ISF | ICR | Named for |
|---|---|---|---|
| Rapid analog — lispro, aspart, glulisine | 1800 ÷ TDD | 500 ÷ TDD | "rule of 1800" / "rule of 500" (Walsh) |
| Regular (human) | 1500 ÷ TDD | 450 ÷ TDD | "rule of 1500" (Davidson) |
The smaller numerators for regular insulin reflect its flatter, longer profile — the same TDD buys a slightly stronger per-unit effect on the calculation. Use the row that matches what you actually prescribed; mixing them is the most common arithmetic error here.
Worked example
Adult on a basal-bolus regimen, TDD 50 units/day, using lispro. Pre-dinner BG is 250 mg/dL, target 120, and dinner is about 60 g of carb.
- ISF = 1800 ÷ 50 = 36 mg/dL per unit
- ICR = 500 ÷ 50 = 10 g per unit
- Correction = (250 − 120) ÷ 36 = 130 ÷ 36 ≈ 3.6 units
- Carb coverage = 60 ÷ 10 = 6 units
- Dinner dose = 6 + 3.6 ≈ 9.5–10 units
Round to what the patient's device can actually deliver — half-unit pens go to 9.5, a standard pen or syringe goes to 10.
This is a starting point, not a fixed value
AACE's 2022 comprehensive care guideline is explicit that a computed ISF "should only be viewed as an estimation due to numerous factors that can alter BG." The formulas are dosing conventions, not validated equations — what makes a correction factor correct is the patient's observed response over the following weeks, reviewed as a pattern rather than a single reading.
Direction of error is worth knowing: in a review of pump-dosing studies, TDD-based formulas underestimated bolus requirements by 13–50% in some patients — though that evidence is specifically adults with type 1 diabetes on insulin pumps (CSII), and shouldn't be carried straight over to a type 2 patient on injections.
Insulin stacking — the hypoglycemia trap
The most common way correction dosing causes a low is stacking: a patient rechecks, sees a still-high number, and re-corrects while the previous dose is still working. The glucose hasn't finished falling yet, so the second dose lands on top of insulin already on board and drives the patient below target.
Pumps and AID systems track insulin on board (IOB) and subtract it automatically. Patients on multiple daily injections have no such tracking — the arithmetic is theirs to do, which makes this an explicit teaching point at every visit where you hand someone a correction scale.
How long is insulin still working? AACE 2022 puts the action time of subcutaneous rapid-acting insulin at 4–6 hours, and expects MDI patients to estimate remaining IOB from the last dose using standard curves. Hirsch's NEJM review is blunter about why it matters: insulin aspart "still has significant activity at 300 minutes" — five hours out. Euglycemic clamp data put the pharmacodynamic duration of rapid analogs at roughly 3–7.5 hours.
No guideline sets a minimum outpatient MDI interval. Not ADA, not AACE, not Endocrine Society. The practical rule that follows from the action time: wait at least 4 hours before a full repeat correction, or subtract estimated IOB if you correct sooner. Treat that as derived guidance, not a cited standard.
For scale: commercial pump bolus calculators use an active insulin time of 2–5 hours (commonly ~4–5), open-source AID systems 5–7. That setting is exactly the subtraction an MDI patient is doing in their head, or not doing.
When to reassess the correction factor
- Time of day. One ISF often doesn't hold across the whole day — many patients need a different morning, midday and evening value (dawn-phenomenon insulin resistance being the usual reason the morning one is tighter).
- Weight or activity change. Both move insulin sensitivity, and neither shows up in the formula until you recompute TDD.
- Glucocorticoids. A steroid burst can blunt a correction factor within a day, and the effect reverses when the course ends — reassess at both edges.
- Acute illness or surgery. Sensitivity can shift fast; formula-derived numbers go stale quickest here.
- Renal or hepatic function change. Falling clearance prolongs insulin effect and typically means less insulin, not more.
- Use the CGM. Pattern review across days — not reaction to single readings — is what actually optimizes a correction factor in anyone wearing a sensor.
Correction is an adjunct, not a regimen
Correction insulin belongs on top of an adequate basal-bolus regimen. The modern term is correctional insulin, which deliberately replaced "sliding scale insulin" — SSI meant reactive dosing given without regard to meal timing, basal insulin, or the individual patient's sensitivity. Correctional insulin is the opposite: sensitivity-derived, and layered onto a regimen that already exists.
Major guidelines discourage using correction insulin alone, without basal — the one carve-out being mild or stress hyperglycemia in a patient holding BG <180 mg/dL. Persistent need for large corrections is a signal that the basal or prandial doses are wrong; fix those with the TDD Starter rather than escalating the correction scale.
Inpatient: preset scales instead of the formula
In the hospital, correction dosing is often not individualized with the ISF formula above. It's applied from a preset scale chosen by TDD category — coarser and slightly heavier than the computed value, on purpose, because inpatient sensitivity moves fast.
| TDD category | Scale | Approx. dose per 50 mg/dL above target |
|---|---|---|
| Low (<40 units/day) | Low | ~1 unit |
| Moderate (40–80 units/day) | Medium | ~2 units |
| High (>80 units/day) | High | ~3–4 units |
- Target differs from clinic: <180 mg/dL inpatient, vs 100–120 outpatient.
- Start correcting at BG 140–150 mg/dL — an inpatient threshold, not an outpatient one.
- Dosing frequency: every 4–6 hours if NPO, or before meals (~4–5 hours apart) if eating. This is the one place a redosing interval is codified.
- Reassess constantly. Evolving illness, changing nutrition and glucocorticoids move sensitivity within a day — a scale that fit on admission may not fit on day three.
References
- Blonde L, Umpierrez GE, Reddy SS, et al. AACE Clinical Practice Guideline: Developing a Diabetes Mellitus Comprehensive Care Plan — 2022 Update. Endocr Pract. 2022.
- American Diabetes Association Professional Practice Committee. Standards of Care in Diabetes — 2026. Diabetes Care. 2026.
- King AB, Kuroda A, Matsuhisa M, Hobbs T. A Review of Insulin-Dosing Formulas for Continuous Subcutaneous Insulin Infusion (CSII) for Adults With Type 1 Diabetes. Curr Diab Rep. 2016.
- Korytkowski MT, Muniyappa R, Antinori-Lent K, et al. Management of Hyperglycemia in Hospitalized Adult Patients in Non-Critical Care Settings: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2022.
- Gauer RL, Abellada A, Stewart M, Kozloski R. Managing Selected Chronic Conditions in Hospitalized Patients. Am Fam Physician. 2024.
- Hirsch IB. Insulin Analogues. N Engl J Med. 2005.
- Heise T, Meneghini LF. Insulin Stacking Versus Therapeutic Accumulation: Understanding the Differences. Endocr Pract. 2014.
- De Block CEM, Van Cauwenberghe J, Bochanen N, Dirinck E. Rapid-acting insulin analogues: theory and best clinical practice in type 1 and type 2 diabetes. Diabetes Obes Metab. 2022.
- Braune K, Lal RA, Petruželková L, et al. Open-source automated insulin delivery: international consensus statement and practical guidance for health-care professionals. Lancet Diabetes Endocrinol. 2022.
mL, pens and vials for a 30- or 90-day Rx
Insulin Product
Daily Dose
Days Supply
Product choice, biosimilars, sig, supplies, safety
The 30-second version
- Basal of choice: pick the interchangeable glargine — insulin glargine-yfgn (Semglee). The pharmacy can swap it for whatever glargine the plan actually covers.
- Leave DAW / "dispense as written" unchecked. Checking it defeats the whole point.
- Write the full sig: units, route, frequency, timing — plus indication.
- Send the pen needles or syringes on a separate Rx, and test strips + lancets if you want a titration log.
- Size quantity to the in-use expiry, not just the units — low-dose patients discard half-full pens. Use the Insulin Quantity calculator.
Glargine: Lantus vs Basaglar vs Semglee vs Rezvoglar
All are insulin glargine 100 units/mL, same amino acid sequence, comparable PK/PD, and no clinically meaningful difference in A1c, hypoglycemia, or immunogenicity. Conversion between them is 1:1. The only difference that matters at the keyboard is FDA regulatory status, because that decides whether the pharmacist is allowed to substitute.
| Product | FDA status | Pharmacy substitution |
|---|---|---|
| Lantus (Sanofi) | Reference biologic | Is the reference — an interchangeable may be swapped in for it |
| Semglee — glargine-yfgn (Viatris/Biocon) | Interchangeable biosimilar | Yes — pharmacist may substitute without calling you (per state pharmacy law) |
| Rezvoglar — glargine-aglr (Lilly) | Interchangeable biosimilar | Yes — same as above; the fallback if a plan prefers Lilly |
| Basaglar (Lilly) | Follow-on biologic (not interchangeable) | No — needs a brand-new Rx from you |
The EHR order-entry trap
Searching glargine in most EHR order-entry screens and taking the first hit usually transmits
Lantus — the reference brand — which on many formularies now sits on a non-preferred tier
and kicks off a prior auth. "Write generically" is not enough when the EHR resolves generic to a brand.
- Search the suffixed name —
insulin glargine-yfgn— and select that product explicitly. - Confirm the transmitted line says glargine-yfgn / Semglee before signing.
- Uncheck DAW so the pharmacy may substitute to the covered glargine.
- If a plan hard-prefers Basaglar, no substitution is possible — you must send a new Rx.
Substitution authority for interchangeables is set by state pharmacy law. Louisiana's rule is below; check your own state before promising a patient the swap happens automatically.
Louisiana substitution law
FDA interchangeability is the federal trigger; state law decides what the pharmacist may then do. Semglee was approved as the first interchangeable biosimilar in July 2021, which is what activates the Louisiana provisions below.
- Substitution is authorized. La. R.S. 37:1164(18)(b)(i) defines "equivalent drug product" to include a biological rated interchangeable in the FDA Purple Book — so an interchangeable biosimilar is legally an equivalent of the reference product. Framework established by HB 319 (2015), effective 8/1/2015.
- You can block it. La. Admin. Code tit. 46, §LIII-2517(B)(1): no interchange when the prescriber prohibits it — handwritten DAW, a brand name given verbally, or DAW / "Brand Medically Necessary" electronically.
- The patient must consent. §LIII-2517(B)(3) permits the interchange only if the patient has been informed of and consented to it. Worth a sentence at the visit — "the pharmacy may fill this as Semglee or another glargine; it's the same insulin" — so the swap isn't a surprise at the counter.
- You get told what was dispensed. §LIII-2517(B)(4): the pharmacist must communicate the product name and manufacturer to you within five business days. Reconcile the med list against that, since the chart will still say what you sent.
- No notice required when you prohibited interchange, when nothing is rated interchangeable/therapeutically equivalent, or on a refill unchanged from the prior fill — so silence does not mean the brand was dispensed.
Rule promulgated under La. R.S. 37:1182; §LIII-2517 amended LR 43:2162 (11/1/2017) and LR 46:793 (6/1/2020). Congress is considering the Biosimilar Red Tape Elimination Act, which would deem biosimilars interchangeable on licensure and collapse this distinction — not law as of July 2026.
Why this is worth the extra clicks
- Biosimilar glargine list price is well below Lantus; real-world spending fell from roughly $191 to $147 per patient per month after biosimilar entry, with no change in outcomes.
- Interchangeable designation drove a large jump in Semglee's formulary placement and dispensing — most commercial and Medicaid plans now cover an interchangeable glargine on a preferred tier.
- Repeated switching between Semglee and Lantus (INSTRIDE 3) showed no change in A1c, FPG, or adverse events; meta-analysis of switching trials shows therapeutic equivalence.
Switching between basal products
- Glargine ↔ glargine (any brand/biosimilar): 1:1, no dose change.
- Tight control or hypoglycemia risk: consider a 10–20% dose reduction on any switch and recheck in ~1 week.
- NPH → glargine/detemir/degludec: reduce total basal ~20% (especially if NPH was BID), then titrate.
- U-300 glargine (Toujeo) is not 1:1 in practice — it is less potent unit-for-unit than U-100; expect to titrate up, and never assume a straight swap.
- Re-teach the device whenever the product changes — pen mechanics and dose windows differ.
Starting insulin in T2DM
- Basal first. Start 10 units daily or 0.1–0.2 units/kg/day; use the higher end with marked hyperglycemia or A1c far above goal. See the TDD starter.
- Titrate to fasting glucose — increase ~2 units every 3 days (or per a written patient algorithm) until fasting is at target. Give the patient the titration rule in writing; untitrated basal is the most common failure.
- Continue metformin and keep a GLP-1 RA / SGLT2i on board where indicated — they lower the insulin dose required and carry cardiorenal benefit.
- Over-basalization: basal above ~0.5 units/kg/day, large bedtime-to-morning glucose swings, or hypoglycemia means stop pushing basal and add prandial coverage or a GLP-1 RA instead.
- Prandial: start 4 units (or 10% of basal) at the largest meal, titrate, then add remaining meals as needed. See the correction factor / ICR tool.
Writing the Rx so it fills
- Sig: "Inject __ units subcutaneously once daily at bedtime" — no "as directed"; pharmacies and plans reject it and days-supply cannot be computed.
- Quantity by container, not by units. Pens come 5 per box (3 mL each); vials are 10 mL. A 30-day supply must round to whole containers.
- In-use expiry drives quantity at low doses. Most opened pens/vials are discarded 28 days after first use (Tresiba/Toujeo 56 days). A patient on 10 units/day still needs a fresh container monthly, even though the units would last far longer — say so in the Rx note so the plan doesn't reject as "early refill."
- Supplies are separate prescriptions: pen needles (e.g. 32G × 4 mm, 1 per injection) or U-100 syringes matched to dose volume; plus test strips and lancets with a frequency that matches the regimen (Medicare ties strip quantity to insulin use).
- Refills: send 90 days when the regimen is stable and the plan allows — fewer gaps, fewer stockouts.
- Glucagon for anyone on insulin with hypoglycemia risk — nasal or auto-injector, and confirm the caregiver knows where it is.
Safety
- Write "units" in full — never "U" or "IU" (10U reads as 100).
- U-500 regular is 5× concentrated: dedicated U-500 syringe or pen only, never a U-100 syringe, and independent double-check of the dose.
- Never share pens between patients even with a new needle.
- Basal insulin does not stop during illness or fasting in T1DM; hold or reduce prandial instead. Give sick-day rules in writing.
- CKD / eGFR falling / weight loss / poor intake: insulin clearance drops — reduce dose proactively rather than after a hypoglycemic event.
- Older adults and limited life expectancy: loosen the A1c target and de-intensify; a "normal" A1c on insulin is a hypoglycemia signal, not a win.
- Confirm the patient can see the dose window, hear the clicks, and physically hold the pen before assuming adherence.
Sources
- Marrison ST, Bragg S, Tran E. Type 2 Diabetes: Outpatient Insulin Management. Am Fam Physician. 2026.
- ADA Professional Practice Committee. 9. Pharmacologic Approaches to Glycemic Treatment: Standards of Care in Diabetes—2026. Diabetes Care. 2026.
- Matli MC, Wilson AB, Rappsilber LM, et al. The First Interchangeable Biosimilar Insulin: Insulin Glargine-Yfgn. J Diabetes Sci Technol. 2023.
- Watanabe JH, Strand MW, Shen W, et al. Insulin Glargine Utilization and Spending Before and After the First Biosimilar Insulin Glargine. J Gen Intern Med. 2025.
- Kakani P, Katcher BA, Maini L. Improved Insurance Coverage Increased Biosimilar Semglee's Market Share After the FDA's Interchangeability Designation. Health Aff. 2025.
- Murphy SJ, Holtkamp NC. Prescription Dispensing for Insulin Glargine After Interchangeable Biosimilar Designation. JAMA Health Forum. 2025.
- Xing X, Zhao L, Wang K, et al. Therapeutic equivalence and switching between biosimilar and reference insulins: systematic review and meta-analysis. Diabetes Obes Metab. 2026.
- La. Admin. Code tit. 46, pt. LIII, §2517 — Prescription Dispensing; Equivalent Drug Product Interchange. La. R.S. 37:1164(18), 37:1182. 2015 La. Acts (HB 319).
Printable standalone page: Insulin Prescribing Best Practices