# Diabetes Type 1 vs Type 2: USMLE Comparison

> What is the difference between type 1 and type 2 diabetes on USMLE?

Antibodies, HLA, C-peptide and twin data that split type 1 from type 2, with ADA 2026 criteria, the 2024 DKA and HHS rules, drug choices and complications.

Type 1 diabetes is T-cell-mediated autoimmune destruction of beta cells, usually causing absolute insulin deficiency: a lean young patient with islet autoantibodies, low C-peptide and ketoacidosis risk who needs insulin from diagnosis. Type 2 diabetes is progressive beta-cell failure on a background of insulin resistance, typically in an adult with obesity; C-peptide persists and ketoacidosis is uncommon.

## What is the difference between type 1 and type 2 diabetes on USMLE?

The difference lies in why insulin fails. In type 1 diabetes, T cells infiltrate the islets (insulitis) and destroy beta cells, so insulin secretion collapses; the ADA Standards of Care in Diabetes--2026 count latent autoimmune diabetes in adults (LADA) as type 1. In type 2 diabetes, insulin resistance in liver and muscle sets the background, and nonautoimmune beta-cell failure progresses over years.

Expect a lean young patient with polyuria, polydipsia, weight loss and often DKA for type 1, and an adult with obesity, acanthosis nigricans and an abnormal screening A1C for type 2. Type 1 clusters with other autoimmune disease, so ADA advises checking for autoimmune thyroid disease soon after diagnosis (see [thyroid function test patterns](/blog/thyroid-disorders-usmle)). The [Endocrinology](/topics/endocrinology) hub covers related topics.

## Type 1 vs type 2 diabetes comparison chart

Eleven features separate the two diseases on a typical stem.

| Feature | Type 1 diabetes | Type 2 diabetes |
| --- | --- | --- |
| Mechanism | T-cell-mediated beta-cell destruction; absolute insulin deficiency | Insulin resistance plus progressive, nonautoimmune beta-cell failure |
| Autoantibodies | GAD65, IA-2, insulin and ZnT8; may disappear after onset | Negative, but 5-10% of adults labeled type 2 test positive (LADA) |
| HLA | DR3-DQ2.5 or DR4-DQ8 in over 90% of patients under 30; DR3/DR4 heterozygotes at highest risk; DR15-DQ6 protective | No HLA link; polygenic, TCF7L2 the strongest common variant |
| Twins and families | Identical-twin concordance about 50%; sibling risk about 8% | Higher in identical than fraternal twins; heritability up to 72% |
| Age and habitus | Often childhood or adolescence, lean, recent weight loss; any age possible | Usually adults, rising with age; obesity, acanthosis nigricans |
| Onset | Acute; DKA is a common first presentation | Gradual; often found by screening |
| C-peptide | Low or undetectable once established | Normal or high early, falling late |
| Acute crisis | DKA | HHS; DKA is less common but occurs (ketosis-prone type 2, SGLT2 inhibitors) |
| Islet histology | Insulitis: T-cell infiltrate, then beta-cell loss | Islet amyloid polypeptide (amylin) deposits |
| Associations | Autoimmune thyroid disease, celiac disease | Hypertension, dyslipidemia, MASLD, polycystic ovary syndrome |
| First treatment | Basal-bolus insulin by injections or pump | Lifestyle plus metformin, a GLP-1 RA or an SGLT2 inhibitor by comorbidity; insulin if A1C is above 10% or glucose >=300 mg/dL |

## LADA, MODY and secondary diabetes

Two groups defy the textbook split: adults with slowly evolving autoimmune diabetes and young people with single-gene diabetes.

| Form | Clues | Management |
| --- | --- | --- |
| LADA | Adult labeled type 2 with islet antibodies, weight loss or early insulin need | Classified as type 1; plan for insulin |
| GCK-MODY | Mild, stable fasting hyperglycemia; OGTT rise under 54 mg/dL; affected parent | Usually no drug treatment outside pregnancy |
| HNF1A- or HNF4A-MODY | Lean, antibody-negative young patient with diabetes in successive generations; HNF1A causes glycosuria at lower glucose, HNF4A macrosomia and neonatal hypoglycemia | Sensitive to sulfonylureas |
| Neonatal diabetes (KCNJ11, ABCC8) | Onset before 6 months of age | Genetic testing; sulfonylurea-responsive |
| Pancreatic diabetes | Acute or chronic pancreatitis, cystic fibrosis | Screen 3-6 months after acute pancreatitis, then yearly |
| Drug-induced | Glucocorticoids, second-generation antipsychotics, checkpoint inhibitors | Checkpoint-inhibitor diabetes can present as DKA |

Suspect monogenic diabetes with negative antibodies, no obesity and an autosomal dominant family history; NIDDK puts monogenic forms at 1 to 5 of every 100 people with diabetes, often misdiagnosed as type 1 or type 2. KCNJ11 and ABCC8 build the beta-cell ATP-sensitive potassium channel, and ABCC8 encodes SUR1, the sulfonylurea receptor, so those infants respond to sulfonylureas.

## Why type 1 makes ketones and type 2 usually does not

Insulin's brake on fat breakdown is the hinge between the two crises. With almost no insulin, adipocytes release free fatty acids, and glucagon-driven hepatic oxidation turns them into acetoacetate and beta-hydroxybutyrate, an anion gap acidosis (see [ketogenesis for Step 1](/blog/2026-06-26-everything-you-need-to-know-about-ketogenesis-for-step-1)). In type 2 diabetes, residual insulin minimizes ketosis without controlling glucose, which climbs over days until osmotic diuresis produces HHS.

Type 2 beta cells also lose first-phase and pulsatile insulin release and secrete more proinsulin. In type 1, a temporary honeymoon phase of near-normal glucose can follow the start of insulin and does not change the diagnosis.

## Diagnostic criteria and screening under ADA 2026

Diabetes is diagnosed on A1C or plasma glucose; unless hyperglycemia is unequivocal (a crisis, or classic symptoms with random glucose >=200 mg/dL), one abnormal result is not enough.

| Test | Diabetes | Prediabetes |
| --- | --- | --- |
| A1C (NGSP-certified laboratory method) | >=6.5% | 5.7-6.4% |
| Fasting plasma glucose (no calories for 8 hours) | >=126 mg/dL | 100-125 mg/dL |
| 2-hour glucose on a 75 g OGTT | >=200 mg/dL | 140-199 mg/dL |
| Random glucose with classic symptoms or hyperglycemic crisis | >=200 mg/dL | -- |

Confirm with two different tests drawn together, such as A1C plus fasting glucose, or one test repeated later. Switch to glucose criteria when the A1C-glucose relationship is altered: some hemoglobin variants, pregnancy, G6PD deficiency, HIV and changes in red-cell turnover such as blood loss, transfusion, hemolysis, erythropoietin or dialysis. Iron deficiency can push A1C falsely high.

ADA 2026 advises testing adults of any age with overweight or obesity (BMI >=25, or >=23 with Asian ancestry) plus a risk factor, such as a first-degree relative with diabetes, hypertension or polycystic ovary syndrome, and everyone else from age 35, repeating at least every 3 years. The USPSTF (2021, grade B) recommends screening adults aged 35-70 with overweight or obesity.

## Type 1 diabetes stages and teplizumab

Islet autoimmunity is detectable before symptoms, so ADA stages type 1 diabetes by antibodies and glucose.

- **Stage 1:** two or more islet autoantibodies, normal glucose, no symptoms.
- **Stage 2:** autoantibodies plus dysglycemia (fasting glucose 100-125 mg/dL, 2-hour OGTT glucose 140-199 mg/dL, or A1C 5.7-6.4% or a rise of 10% or more), no symptoms.
- **Stage 3:** diabetes by standard criteria, usually symptomatic.

ADA 2026 recommends offering autoantibody screening to relatives of people with type 1 diabetes and others at known genetic risk. Under its June 2026 label, teplizumab (Tzield), a daily-infused anti-CD3 monoclonal antibody, delays stage 3 in patients aged 1 year and older with stage 2 disease (median 50 versus 25 months to stage 3 with placebo); in children aged 8-17 recently diagnosed with stage 3, it delays the decline in insulin production. A new boxed warning covers serious Epstein-Barr virus and cytomegalovirus reactivation; cytokine release syndrome and lymphopenia are other labeled risks. ADA 2026, written earlier, still cites age 8.

## DKA vs HHS: 2024 consensus criteria and management

A 2024 consensus from ADA, EASD, JBDS, AACE and DTS redefined both crises, and the ADA 2026 Standards adopt its criteria.

| Feature | DKA | HHS |
| --- | --- | --- |
| Glucose | >=200 mg/dL or known diabetes (older cut-off 250) | >=600 mg/dL |
| Ketones | Beta-hydroxybutyrate >=3.0 mmol/L or urine ketones 2+ or more | Beta-hydroxybutyrate under 3.0 mmol/L or urine ketones under 2+ |
| Acid-base | pH under 7.3 and/or bicarbonate under 18 mmol/L | pH >=7.3 and bicarbonate >=15 mmol/L |
| Osmolality | Not part of the definition | Effective above 300 or total above 320 mOsm/kg |
| Course | Hours to days; vomiting, abdominal pain, Kussmaul breathing | Days to a week; altered mental status common |
| Starting IV insulin | Regular insulin 0.1 units/kg/h; subcutaneous analog acceptable in mild DKA | Regular insulin 0.05 units/kg/h |
| Glucose target after dextrose | 150-200 mg/dL | 200-250 mg/dL |
| Resolution | Ketones under 0.6 mmol/L and venous pH >=7.3 or bicarbonate >=18 | Osmolality under 300, urine output above 0.5 mL/kg/h, glucose under 250, clearer mentation |

- **Fluids first:** isotonic saline or a balanced crystalloid such as lactated Ringer's, 500-1000 mL/h for the first 2-4 hours.
- **Potassium:** once urine output is confirmed, below 3.5 mmol/L give 10-20 mmol/h and hold insulin until it exceeds 3.5 (older references use 3.3); at 3.5-5.0, add potassium to the fluids to keep it at 4-5; above 5.0, start insulin without it.
- **Insulin:** a fixed-rate infusion; once glucose falls below 250 mg/dL, add 5-10% dextrose and cut the DKA rate to 0.05 units/kg/h rather than stopping.
- **Bicarbonate** only below pH 7.0; **phosphate** only with weakness or respiratory compromise and a level under 1.0 mmol/L.
- **Transition:** overlap subcutaneous insulin with the drip for 1-2 hours.
- **Euglycemic DKA:** about 10% of DKA has glucose under 200 mg/dL, classically with SGLT2 inhibitors around surgery, fasting or illness; give dextrose with insulin from the start.

> **10-second crisis check:** ketones plus acidosis with glucose >=200 (or known diabetes) is DKA; glucose >=600 with effective osmolality above 300 and pH >=7.3 is HHS. Fluids first, potassium above 3.5 before insulin, dextrose at 250, and stop the drip only when ketones and pH clear.

DKA mortality is now under 1%, HHS mortality is five- to tenfold higher, and about one-third of crises are hybrids. Practice the answer-choice logic with this [DKA vs HHS question breakdown](/blog/2026-06-09-q-bank-breakdown-dka-vs-hhs-why-every-answer-choice-matters).

## Insulin types: onset, peak and duration

Insulin questions turn on onset, peak and which preparation runs in an IV line.

| Insulin | Onset | Peak | Duration | Main use |
| --- | --- | --- | --- | --- |
| Rapid-acting: lispro, aspart, glulisine | 5-15 min | 45-75 min | 3-5 h | Meals, corrections, pumps |
| Short-acting: regular | 30-60 min | 2-4 h | 6-8 h | IV infusion in DKA, HHS and hospital |
| Intermediate: NPH | About 2 h | 4-12 h | 18-26 h | Basal; more hypoglycemia than analogs |
| Long-acting: glargine, degludec | 1-4 h | No peak | 24 h (degludec over 40 h) | Once-daily basal |
| Weekly basal: icodec | 6 h | 2-4 days | 7 days or more | Adults with type 2 diabetes only |

FDA approved two once-weekly basal insulins in 2026, icodec (Awiqli) in March and efsitora alfa (Onswik) in September, both only for adults with type 2 diabetes.

## Non-insulin drugs: mechanism, adverse effects and best fit

Each non-insulin class pairs one mechanism with a signature adverse effect and a preferred patient.

| Class (example) | Mechanism | Signature adverse effects | Best fit |
| --- | --- | --- | --- |
| Metformin | Suppresses hepatic glucose output | GI upset, B12 deficiency; boxed lactic acidosis warning; contraindicated below eGFR 30 | Low cost, no hypoglycemia, weight neutral |
| Sulfonylureas (glipizide) | Act on SUR1 of the beta-cell K-ATP channel to release insulin | Hypoglycemia, weight gain | Low cost; HNF1A- or HNF4A-MODY |
| DPP-4 inhibitors (sitagliptin) | Slow inactivation of endogenous GLP-1 and GIP | Pancreatitis, arthralgia, bullous pemphigoid; heart failure signal with saxagliptin | Oral, weight neutral; not with a GLP-1 RA |
| GLP-1 RAs (semaglutide); tirzepatide adds GIP agonism | Glucose-dependent insulin release, less glucagon, slower gastric emptying | Nausea, gallbladder disease, pancreatitis; boxed thyroid C-cell tumor warning, so avoid with MTC or MEN 2 | ASCVD, CKD, obesity, HFpEF with obesity; first injectable before insulin |
| SGLT2 inhibitors (empagliflozin) | Block proximal-tubule glucose reabsorption | Genital mycotic infection, urosepsis, volume depletion, Fournier gangrene, euglycemic DKA | Heart failure, CKD (start at eGFR >=20), ASCVD |
| Pioglitazone | PPAR-gamma agonist; raises insulin sensitivity | Edema and heart failure (boxed), fractures, bladder cancer | MASH; avoid in heart failure |
| Pramlintide | Amylin analog: slower gastric emptying, less glucagon, satiety | Severe hypoglycemia with insulin (boxed) | Add-on to mealtime insulin; FDA now lists Symlin as discontinued |

ADA 2026 chooses type 2 therapy by comorbidity, irrespective of A1C: ASCVD calls for a GLP-1 RA and/or an SGLT2 inhibitor with proven benefit, heart failure for an SGLT2 inhibitor, obesity with symptomatic HFpEF for tirzepatide or a GLP-1 RA, and CKD for an SGLT2 inhibitor or a GLP-1 RA with kidney benefit, preferring the GLP-1 RA once eGFR is under 30. Metformin remains the usual first oral drug in classic teaching, but ADA now lists it as one option rather than a required step. Its toxicities sit among the ranked [high-yield drug side effects](/blog/high-yield-drug-side-effects-usmle).

## Chronic complications and their mechanisms

Chronic hyperglycemia injures tissue through advanced glycation end products, the polyol pathway, protein kinase C activation and oxidative stress. Peripheral nerve takes up glucose without insulin through GLUT1 and GLUT3, so aldose reductase converts the excess to sorbitol, consuming the NADPH that regenerates glutathione; sorbitol accumulates and oxidative stress follows.

- **Nephropathy:** hyperfiltration, then basement membrane thickening, mesangial expansion, nodular Kimmelstiel-Wilson lesions and hyalinosis of both arterioles. Check UACR and eGFR yearly (type 2 from diagnosis, type 1 after 5 years). An ACE inhibitor or ARB, which relaxes the efferent arteriole, treats albuminuria with hypertension, tolerating a creatinine rise up to 30%; add an SGLT2 inhibitor from eGFR 20, a GLP-1 RA with kidney benefit and, at eGFR >=25, finerenone, checking potassium at 1 month.
- **Retinopathy:** microaneurysms progress to neovascularization and macular edema. The first dilated exam comes at diagnosis in type 2 and 5 years after onset in type 1, then yearly (every 1-2 years if normal with glucose at goal); anti-VEGF injections lead for center-involving macular edema and laser treats high-risk proliferative disease.
- **Neuropathy:** stocking-glove sensory loss plus gastroparesis, orthostatic hypotension or resting tachycardia. Test with a 10-g monofilament yearly; treat pain with gabapentinoids, SNRIs, tricyclics or sodium channel blockers, not opioids.
- **Macrovascular disease:** coronary, cerebrovascular and peripheral artery disease; adults with diabetes aged 40-75 get at least a moderate-intensity statin, high intensity with ASCVD or added risk factors.

## How USMLE tests type 1 vs type 2 diabetes

Most stems hide one discriminating clue in the history, the antibodies or a single lab value.

- **Lean 13-year-old, vomiting, deep rapid breathing, glucose 480 mg/dL, high beta-hydroxybutyrate:** type 1 diabetes in DKA; fluids and a potassium check before insulin.
- **Confused nursing-home resident, glucose 950 mg/dL, effective osmolality 340, pH 7.38:** HHS; fluids, then insulin at a lower rate.
- **Day 2 after surgery on empagliflozin, glucose 160 mg/dL, pH 7.15, high ketones:** euglycemic DKA; insulin plus dextrose, and stop the SGLT2 inhibitor.
- **Nurse with glucose 38 mg/dL, high insulin, low C-peptide and low proinsulin:** injected insulin; an insulinoma or a sulfonylurea raises C-peptide.

## Worked vignette: the adult who is not type 2

A 41-year-old man diagnosed with type 2 diabetes 9 months ago takes metformin faithfully, yet his A1C has risen from 7.4% to 10.6%, with thirst, frequent urination and an unintentional 5 kg weight loss. His BMI is 22 kg/m², and he has no acanthosis nigricans. GAD65 antibodies are strongly positive, and fasting C-peptide is low. Which is the most appropriate next step in management?

- A. Add glipizide
- B. Add empagliflozin
- C. Start basal-bolus insulin
- D. Begin a teplizumab infusion course
- E. Add sitagliptin

**Answer: C.** A lean adult with GAD65 antibodies, low C-peptide, weight loss and early failure of oral therapy has latent autoimmune diabetes in adults, which ADA classifies as type 1, and an A1C above 10% with symptoms calls for insulin in any case. Glipizide (A) and sitagliptin (E) depend on failing beta cells, and sitagliptin's label excludes type 1 diabetes. Empagliflozin (B) is not recommended in type 1 diabetes because it raises the risk of ketoacidosis. Teplizumab (D) delays stage 3 in stage 2 patients and, after a stage 3 diagnosis, is approved only for children aged 8-17.

## Common wrong-answer traps

- **Assuming adult onset means type 2.** Type 1 diabetes can begin at any age, and 5-10% of adults labeled type 2 carry islet antibodies.
- **Reading a preserved C-peptide as proof of type 2.** C-peptide can still be normal early in type 1 disease; antibodies decide.
- **Stopping the insulin drip once glucose normalizes.** DKA resolves by ketones and pH or bicarbonate, so insulin continues with dextrose.
- **Trusting A1C after hemorrhage, transfusion or hemolysis.** Use plasma glucose criteria instead.

## Sources

- American Diabetes Association: [Standards of Care in Diabetes--2026](https://professional.diabetes.org/standards-of-care) (sections 2, 3, 4, 9, 10, 11, 12 and 16)
- Endotext (NCBI Bookshelf): [Hyperglycemic crises, with the 2024 consensus criteria](https://www.ncbi.nlm.nih.gov/books/NBK279052/)
- NIDDK: [A1C test](https://www.niddk.nih.gov/health-information/diagnostic-tests/a1c-test), [monogenic diabetes](https://www.niddk.nih.gov/health-information/diabetes/overview/what-is-diabetes/monogenic-neonatal-mellitus-mody); Diabetes in America: [classification and diagnosis](https://www.niddk.nih.gov/-/media/Files/Strategic-Plans/Diabetes-in-America-3rd-Edition/DIA_Ch01.pdf), [genetics of type 1 diabetes](https://www.niddk.nih.gov/-/media/Files/Strategic-Plans/Diabetes-in-America-3rd-Edition/DIA_Ch12.pdf), [genetics of type 2 diabetes](https://www.niddk.nih.gov/-/media/Files/Strategic-Plans/Diabetes-in-America-3rd-Edition/DIA_Ch14.pdf)

Turn antibodies, crisis criteria and drug choices into reflexes before test day: [Practice endocrinology questions on StepGenie](https://dashboard.stepgenie.app/sign-up).

## Frequently asked questions

### How do you tell type 1 from type 2 diabetes on a USMLE question?

Type 1 diabetes is usually a lean child or young adult with weight loss, polyuria and often DKA, islet autoantibodies (GAD65, IA-2, insulin or ZnT8), low C-peptide and HLA-DR3 or DR4. Type 2 is an adult with obesity, acanthosis nigricans, a screening A1C, negative antibodies and preserved C-peptide. In adults with mixed features, autoantibodies decide, because C-peptide can still be normal early in type 1 and can fall late in type 2.

### What are the ADA 2026 criteria for diagnosing diabetes?

Any one of four results diagnoses diabetes: an A1C of 6.5% or higher, a fasting plasma glucose of 126 mg/dL or higher after 8 hours without calories, a 2-hour glucose of 200 mg/dL or higher on a 75 g oral glucose tolerance test, or a random glucose of 200 mg/dL or higher with classic symptoms or a hyperglycemic crisis. Without unequivocal hyperglycemia, confirm with two abnormal results, either two different tests drawn together or one test repeated.

### How are DKA and HHS defined under the 2024 consensus?

DKA requires glucose of 200 mg/dL or more or known diabetes, beta-hydroxybutyrate of 3.0 mmol/L or more or urine ketones of 2+ or more, and a pH below 7.3 and/or bicarbonate below 18 mmol/L. HHS requires glucose of 600 mg/dL or more, effective osmolality above 300 or total osmolality above 320 mOsm/kg, beta-hydroxybutyrate below 3.0 mmol/L, and a pH of 7.3 or more with bicarbonate of 15 mmol/L or more.

### What is the difference between LADA and MODY?

LADA is autoimmune diabetes that begins in adulthood: islet antibodies such as GAD65 are present, beta-cell function declines, and ADA classifies it as type 1 diabetes, so clinicians plan for insulin. MODY is monogenic, autosomal dominant and antibody-negative, usually in a lean teen or young adult. GCK-MODY causes mild, stable fasting hyperglycemia that usually needs no drug treatment outside pregnancy, while HNF1A- and HNF4A-MODY respond to sulfonylureas.

### Who can receive teplizumab for type 1 diabetes in 2026?

Teplizumab (Tzield) is an anti-CD3 monoclonal antibody given by daily IV infusion. Its June 2026 FDA label covers people aged 1 year and older with stage 2 type 1 diabetes, meaning two or more islet autoantibodies plus dysglycemia without symptoms, to delay stage 3, and children aged 8 to 17 recently diagnosed with stage 3 disease, to delay the decline in insulin production. It carries a boxed warning for Epstein-Barr virus and cytomegalovirus reactivation.

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Canonical page: [Diabetes Type 1 vs Type 2: USMLE Comparison](https://www.stepgenie.app/blog/diabetes-type-1-vs-type-2-usmle)
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