“HI” and “LO” Blood Sugar Readings Explained: What to Do Next?

It can be a scary and a bit freezing experience when your blood glucometer suddenly shows “HI” or “LO” instead of a number. What does that even mean? Is something really wrong with me? No matter whether you are a newly diagnosed diabetes patient or have been using your blood glucose monitor for years, seeing a HI or LO blood sugar reading can be unnerving.

Don’t panic! This article will help you understand what these letters actually mean and what you should do if you see them.

Decoding “HI” and “LO” Readings

When your meter says “HI” or “LO,” don’t assume that’s an error sign. It’s an alert, warning you that your blood sugar level is outside the glucometer’s measurable range. In most meters, this range is often around 20 to 600 mg/dL, or about 1.1 to 33.3 mmol/L [1].

These readings are essentially danger warnings. If your blood glucose level is higher than the device’s highest limit (about 600 mg/dL or 33.3 mmol/L on standard meters), it will display “HI” to indicate the value is dangerously high. This is commonly interpreted as severe hyperglycemia (a very high blood glucose level) that is beyond the meter’s upper range.

Likewise, if your blood sugar level is below the lowest limit (around 20 mg/dL or 1.1 mmol/L), it will display “LO,” which indicates dangerously low blood sugar, or severe hypoglycemia. Both situations are considered medical emergencies and need immediate attention [1, 2]. 

First Steps: Rule Out a Testing Error

This is the first thing first that you should do, because a false HI or LO reading is highly possible. You may get a testing error due to contaminated fingers or defective strips [3]. So, before treating it as a medical emergency, go through the checklist below to confirm whether the reading you saw reflects your true blood sugar level:

• Wash and dry your hands, then retest.
• Use a fresh test strip from a new vial.
• Insert the strip correctly and obtain the blood sample properly.
• Check whether the test strip has an expiration date.
• Confirm whether the strips were stored improperly (too hot, cold, or humid).
• Retest on a different finger if needed.
• Check whether the meter is calibrated and does not have a low battery issue.

If your blood sugar level continues to show a “HI” or “LO” reading even after you double checked the above checklist, it’s time to focus on your symptoms and take appropriate action [1].

Action Plan: If Your Meter Says “LO”

If you truly have a LO” reading, that simply means you have severe hypoglycemia (extremely low blood sugar) and your condition may rapidly deteriorate and become life-threatening [4]. Here’s a step-by-step guide on what to do:

• Don’t delay, act immediately. Treat it as severe hypoglycemia, which can make you disoriented or even cause you to pass out. Take immediate action to raise your blood sugar.
• Eat 15 grams of fast‑acting carbohydrate. The rule of thumb is the 15/15 rule: have about 15 grams of quick sugar (for example, 3-4 glucose tablets, half a cup of juice or regular soda, a tablespoon of honey, or a few candies) and then wait 15 minutes. 
• Re-check your blood sugar after 15 minutes. Using your meter (with a new strip), test again to see if your level has improved. If it’s still below 70 mg/dL (3.9 mmol/L) or the meter still says “LO,” treat again with another 15 grams of fast carbs. 
• Know when to get medical help. If at any point you feel like you might pass out, have a seizure, or can’t swallow, or if you’ve treated twice and your meter still reads “LO,” call emergency medical services (EMS) right away (or have someone call for you). If available, a trained person should give glucagon injection or intranasal spray. 
  

Remember, severe hypoglycemia is associated with high mortality rate [5]. Repeated “LO” readings mean your actual blood sugar may be critically low, so do not try to “wait it out.” Dial your local EMS number immediately [4, 6].

When the Meter Says “HI”

This message means your hyperglycemia is extremely high and you’re at risk for serious complications like diabetic ketoacidosis (DKA) or hyperosmolar hyperglycemic state (HHS) [7]. Here’s what to do if you see “HI” on your meter:

• Evaluate your symptoms. Extremely high blood sugar can make you feel very unwell. You may have symptoms like nausea or vomiting, extreme thirst and dry mouth, abdominal pain, blurry vision, deep or rapid breathing with a fruity odor (Kussmaul breathing) and altered mental state. These are all possible signs of DKA or HHS. If you are experiencing any of those symptoms, seek urgent medical help [8].
• Follow your diabetes emergency plan (if you have one). If your doctor has given you specific instructions for dealing with very high blood sugar, such as taking a correction dose of insulin or extra medication, follow those right away. 
  Do not engage in strenuous activity or delay care. Such activities can actually make things worse in this state. 
• Do not engage in strenuous activity or delay care. Such activities can actually make things worse in this state. 
  
• Drink plenty of water. High blood sugar causes dehydration, and dehydration in turn can push blood sugar even higher. Start drinking water (or sugar-free fluids like clear broth or tea) in small sips if you feel nauseated. Fluids help dilute the excess glucose and flush it out through urine. 
• Keep checking your blood sugar level and don’t avoid additional symptoms. Take a reading every hour or two if possible to see if it’s trending down. Also keep an eye on your temperature and how you’re feeling overall, since fever or infection can push blood sugar up. Ask for help or call an ambulance if needed.

Tips to Avoid Extreme Highs and Lows

It’s impossible to prepare everyone for every situation, but you certainly can reduce the chances of severe highs or lows with some planning and healthy habits:

• Keep a “low kit” in your purse, backpack or bedside. Pack glucose tablets or a small juice box and spare strips and lancets.
• Label insulin and strip bottles when you open them. Discard as soon as they are expired.
• Have a backup meter and extra batteries, especially when travelling.
• Save your doctor’s and a family member’s numbers as emergency contacts
• Educate family and friends on how to treat hypo‑ and hyperglycemia, including the use of glucagon.

Upgrade to a Reliable Meter and Testing Kit

So what happens if your meter shows HI or LO, but it turns out to be a false reading? Yes, undue stress! And if you want to avoid that, you need a blood glucose monitor that’s dependable and comes with high-quality strips. Sinocare is a global brand trusted by over 25 million users worldwide.

Why choose Sinocare’s meters?

• Precise and reliable. In a 2020 randomized controlled trial, after evaluating five selfmonitoring blood glucose (SMBG) systems, the researchers found that the Sinocare Gold AQ meter had the highest accuracy compared to other tested devices. 
• Simple operation. Sinocare’s Safe AQ series uses code-free strips, requires only a small blood sample (~0.6 µL) and provides results in about 5 seconds.
• Standard measurement range. Safe AQ meters measure glucose from 1.1–33.3 mmol/L (20–600 mg/dL), allowing reliable detection of HI and LO events.
• Global trust. With over 50% diabetics in China using Sinocare products for self-monitoring and a rapidly growing international user base, the brand is recognized for offering reliable, affordable devices.

References

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2. Battelino T, Danne T, Bergenstal RM, Amiel SA, Beck R, Biester T, Bosi E, Buckingham BA, Cefalu WT, Close KL, Cobelli C. Clinical targets for continuous glucose monitoring data interpretation: recommendations from the international consensus on time in range. Diabetes care. 2019 Aug 1;42(8):1593-603. 
3. Mathew TK, Zubair M, Tadi P. Blood Glucose Monitoring. InStatPearls [Internet] 2025 Jan. StatPearls Publishing. 
4.Mathew P, Thoppil D. Hypoglycemia. InStatPearls [Internet] 2022 Dec 26. StatPearls Publishing.
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6. Davis HA, Spanakis EK, Cryer PE, Siamashvili M, Davis SN. Hypoglycemia During Therapy of Diabetes. In Endotext [Internet] 2024 Jul 7. MDText. com, Inc.
7. Mouri M, Badireddy M. Hyperglycemia. InStatPearls [Internet] 2023 Apr 24. StatPearls Publishing.
8. González P, Lozano P, Ros G, Solano F. Hyperglycemia and oxidative stress: an integral, updated and critical overview of their metabolic interconnections. International journal of molecular sciences. 2023 May 27;24(11):9352.
9. Li X, Zheng S, Pan S, Yan T, Di J, Yang Y, Cheng J. Accuracy Evaluation of Five Blood Glucose Monitoring Systems in Patients from Different Departments. Experimental and Clinical Endocrinology & Diabetes. 2020 Apr;128(04):210-5.