Living with diabetes means reading a glucose meter correctly, knowing what an error code is telling you, and understanding a growing list of medicines, from metformin to SGLT2 tablets like Jardiance and weekly GLP-1 pens like Ozempic and Mounjaro. This guide covers accurate testing, meter errors, the main medicine groups, sick-day rules and storing pens.
The short answer
- Meter errors almost always point to the strip, the blood drop, temperature or the battery, but every brand numbers its codes differently, so check your meter’s manual.
- Accuracy: home meters are allowed to differ from a lab result by up to 15% in most readings. Wash and dry your hands, use in-date strips and a good drop of blood.
- Medicines: metformin, SGLT2 inhibitors (Jardiance, Forxiga), DPP-4 inhibitors, sulfonylureas, GLP-1 pens and insulin each have their own side effects and “sick-day” rules.
How accurate is a home glucose meter?
A blood glucose meter measures the glucose in a tiny drop of capillary blood using an enzyme on the test strip. Under the international standard ISO 15197:2013, 95% of readings must fall within ±15% of a laboratory reference above 5.6 mmol/L (100 mg/dL), and within ±0.83 mmol/L (15 mg/dL) below it. So a meter showing 10.0 mmol/L when the lab says 8.8 is still within specification.
Australia, the UK and most of the world use mmol/L; the US uses mg/dL. To convert, multiply mmol/L by 18 (5.5 mmol/L ≈ 99 mg/dL). If your meter suddenly shows numbers like 110 instead of 6.1, it may have been switched to the other unit; most meters fix this in the settings or require a replacement meter.
Getting a reliable reading
- Wash and dry your hands. Fruit juice or sugar on the fingers can raise a reading dramatically. Alcohol wipes are fine if the skin dries first.
- Check the strips: in date, the right strips for the meter, the vial closed between uses, used within the time printed after opening.
- Use the side of the fingertip and let a full, round drop form. Don’t squeeze hard, which dilutes the sample with tissue fluid.
- Keep meter and strips at room temperature. Most meters work only between about 10 °C and 40 °C.
- Use control solution when you open a new box of strips, if the meter is dropped, or if readings don’t match how you feel.
What do glucose meter error codes mean?
There is no universal error code list. “E-4” on one brand can mean the strip was removed too early, while “Er4” on another means the blood sample was too small. The table shows the common categories most manufacturers use.
| Category | Example codes (vary by brand) | Usual fix |
|---|---|---|
| Strip problem (used, damaged, wrong type) | E-1, E-2, Er1, Er3 | Insert a new strip from a closed vial |
| Not enough blood / applied too late | E-3, E-4, Er4, E5 | New strip, larger drop, apply within the time limit |
| Temperature out of range | “Temp” icon, E-T, Er5 | Let the meter sit 20–30 minutes in a room-temperature place |
| Reading too high / too low | HI, LO | Retest; if confirmed, act on it (see box below) |
| Meter or battery fault | E-0, E-6 and above, battery icon | Replace the battery; call the manufacturer helpline |
Brand-specific meanings and step-by-step fixes are in glucose meter error codes (ER4, E-4 and others). Date, time and memory resets are covered in how to reset a glucose meter.
“HI” or a very high reading with vomiting, abdominal pain, fast breathing or drowsiness can mean diabetic ketoacidosis and needs urgent medical care. A low reading (under 4.0 mmol/L or 70 mg/dL) needs 15 g of fast-acting glucose (for example 150 mL of regular soft drink or glucose tablets), a recheck after 15 minutes, and then a snack. Call an ambulance if the person can’t swallow safely or is unconscious.
Diabetes medicines at a glance
| Group | Examples (brands) | How it lowers glucose | Main things to know |
|---|---|---|---|
| Biguanide | Metformin (Diabex, Glucophage, Diaformin) | Less glucose released by the liver | Stomach upset at first; take with food; extended-release is gentler; low B12 with long use |
| SGLT2 inhibitors | Empagliflozin (Jardiance), dapagliflozin (Forxiga, Farxiga) | Glucose passed out in urine | Genital thrush, dehydration; rare ketoacidosis; stop before surgery; also protect heart and kidneys |
| DPP-4 inhibitors | Sitagliptin (Januvia), linagliptin (Trajenta) | Boost the body’s own incretin hormones | Well tolerated; weight-neutral |
| Sulfonylureas | Gliclazide (Diamicron), glimepiride | Make the pancreas release more insulin | Can cause hypos and weight gain |
| GLP-1 receptor agonists | Semaglutide (Ozempic, Rybelsus), dulaglutide (Trulicity), tirzepatide (Mounjaro; a dual GIP/GLP-1 agonist) | More insulin after meals, slower stomach, less appetite | Nausea when doses go up; weight loss; storage rules for pens |
| Insulin | Glargine, degludec, aspart, lispro and others | Replaces or tops up insulin | Dose adjustment, hypos, injection technique, storage |
Myo-inositol is a popular supplement for polycystic ovary syndrome and is sometimes combined with metformin; see metformin and inositol together.
Sick-day rules
During vomiting, diarrhoea or a fever with poor fluid intake, some diabetes and heart medicines can harm the kidneys or cause ketoacidosis. Many diabetes services teach “sick-day rules”: temporarily stop metformin, SGLT2 inhibitors, sulfonylureas (risk of hypo if not eating), diuretics, ACE inhibitors, ARBs and NSAIDs until you are eating and drinking normally for 24–48 hours. Never stop insulin; check glucose more often and ask for advice on doses. Your diabetes team should give you a personal written plan.
Storing Ozempic, Mounjaro and insulin
| Product | Before first use | After first use / out of the fridge |
|---|---|---|
| Ozempic pen | Fridge 2–8 °C | 56 days below 30 °C or in the fridge |
| Mounjaro single-dose pen (US) | Fridge 2–8 °C | Up to 21 days unrefrigerated below 30 °C |
| Mounjaro KwikPen (multi-dose) | Fridge 2–8 °C | 30 days below 30 °C after first use |
| Most insulin pens and vials | Fridge 2–8 °C | About 28 days at room temperature (check the brand) |
Never freeze these medicines or leave them in a hot car. Details and travel tips: how long Ozempic and Mounjaro can stay out of the fridge.
Buying diabetes medicines abroad
High prices lead many people, especially in the US, to buy Jardiance, Ozempic or insulin in Mexico or online. Some genuine products are cheaper abroad, but counterfeit semaglutide pens have been found in several countries, and many “compounded” versions sold online are unregulated. Read buying Jardiance or Ozempic in Mexico before you go.
Articles in this guide
Each article answers one specific question in depth.
- Glucose Meter Error Codes: What ER4, E-4 and Other Errors MeanError codes differ by brand, but most point to the strip, the blood sample or temperature. A troubleshooting table for E-4/ER4 and other common codes.
- How to Reset a Glucose Meter (and When You Shouldn't)Most meters don't have a reset button: you change the battery, set date and time, or clear memory. Steps, control-solution checks, and what not to delete.
- Buying Jardiance or Ozempic in Mexico: Legal and Safety FactsJardiance is sold in Mexican pharmacies, often cheaper. Prescription rules, bringing it home, counterfeit risk with GLP-1 pens, and safer ways to save.
- Storing Ozempic and Mounjaro: How Long Out of the Fridge?Opened Ozempic pens last 56 days below 30 °C; Mounjaro pens 21 days below 30 °C. Storage rules for travel, heat and freezing.
- Can You Take Metformin and Inositol Together?Myo-inositol is popular in PCOS alongside metformin. What studies show, doses used, blood sugar effects and when to talk to your doctor.
Frequently asked questions about diabetes care
What does “E-4” mean on an On Call Sure glucometer?
Error codes depend on the model. On many meters a code in the E-3 to E-5 range points to a sample or strip problem. Check the “Error messages” page of your manual or the manufacturer’s website, then retest with a new strip.
What affects blood sugar besides food and medicine?
Many things: exercise, illness and infection, stress, poor sleep, alcohol, steroids and some other medicines, hormonal changes, heat, dehydration, caffeine in some people, and the “dawn phenomenon” in the early morning. A diary next to your readings helps spot patterns.
Should a fingerstick and a CGM read the same?
Not exactly. Continuous glucose monitors (FreeStyle Libre, Dexcom) measure fluid under the skin, which lags 5–15 minutes behind blood, especially when glucose is changing quickly. Use a fingerstick if symptoms don’t match the sensor.
Can I keep using my meter after its warranty ends?
Yes, if it passes control-solution checks and readings make sense. Manufacturers and diabetes organisations often replace old meters free, so it’s worth asking when strips change.
Sources and further reading
- Diabetes Australia – Blood glucose monitoring
- Diabetes UK – Checking your blood sugar levels
- American Diabetes Association – Hypoglycemia
- US FDA – Blood glucose monitoring devices
- Ozempic (semaglutide) – Consumer Medicine Information / Patient leaflet (Novo Nordisk)
- US FDA – Warning about counterfeit Ozempic
Taking medicinesCan you cut it in half? What if you miss or double a dose? Are expired tablets safe? Practical rules for…
Surgery & proceduresWhich medicines to stop before surgery, what is usually fine, fasting rules, and pain relief, laxatives and…
Vitamins & supplementsVitamin D, K2, CoQ10, omega-3 and magnesium: who may benefit, forms and doses, interactions with medicines, and how…