Understanding How Illness Disrupts Glucose Control and How OpenAPS Helps

When you are sick, your body mounts a stress response that directly interferes with glucose metabolism. Stress hormones such as cortisol, adrenaline, and growth hormone are released to fight infection, but they also block insulin from moving glucose into cells. This leads to a rise in blood glucose even if you have eaten little or nothing. At the same time, many illnesses cause reduced appetite, delayed gastric emptying, or vomiting and diarrhea, which can create unpredictable swings into hypoglycemia. The result is a volatile and dangerous combination that even the most diligent manual management struggles to tame.

OpenAPS (Open Artificial Pancreas System) is designed to handle such chaos by continuously adjusting insulin delivery based on CGM readings. During illness, it automatically modifies basal rates and issues temporary basals to prevent prolonged highs without waiting for manual input. However, optimal performance requires you to understand how OpenAPS interprets your current condition and to make smart adjustments to your system in advance. This guide will walk you through every step—from pre‑illness preparations to recovery—so you can navigate sick days with greater confidence.

Preparing Your OpenAPS System Before You Get Sick

Preparation is the single most important step for successful sick-day management. Because OpenAPS is a DIY system, you have complete control over its configuration—and with that control comes the responsibility to set up appropriate guardrails before you become ill. A few hours of proactive work can prevent days of chaos.

Stockpile Supplies and Confirm Hardware Function

  • CGM sensors and transmitters: Have at least two spare sensors and a fully charged transmitter. Illness often causes rapid swings, so sensor accuracy is critical. Check that your transmitter is within its warranty period and that you have adhesive patches or overtape to re‑secure a failing sensor.
  • Insulin pump supplies: Reservoirs, infusion sets, and batteries. Ensure your pump’s battery is fresh and you have a backup vial of rapid‑acting insulin. Consider using a longer infusion set cannula (e.g., 9 mm instead of 6 mm) if you experience increased resistance during fever. Always have a spare infusion set ready to swap if a site fails.
  • Ketone test strips: Blood ketone strips (preferred) or urine strips. Aim to have a fresh vial available. Blood strips are more accurate and give earlier warning of rising ketones. If you use urine strips, remember they lag behind blood levels by several hours.
  • Glucose tablets or gel: For emergency hypoglycemia treatment, especially if vomiting makes it hard to eat. Glucose gel can be absorbed through the gums when you cannot swallow. Store multiple packs in different locations (nightstand, bag, car).
  • Backup plan: Identify a family member or friend who understands your OpenAPS setup. Show them how to check CGM data, how to change infusion sets, and where your glucagon kit is kept. Write down simple steps and tape them to your pantry door.

Review and Adjust Your OpenAPS Settings

Before illness hits, review your target glucose range, maximum bolus duration, and max IOB (insulin‑on‑board). During sickness, a slightly higher target (e.g., 120–140 mg/dL instead of 100–110) can provide a safety margin against hypoglycemia. Many OpenAPS users also set a temporary target of 130–150 mg/dL at the first sign of illness. Additionally, enable the autosensitivity feature if you haven’t already—it helps OpenAPS detect when you are more insulin resistant due to illness and automatically adjusts dosing.

Check your maximum basal rate and how often the system can issue temporary basals. Consider lowering your max IOB fraction in settings (from 0.8 to 0.6) to reduce stacking risk if you plan to rely on temp targets. Do not forget to review your low‑glucose suspend threshold—set it slightly higher (e.g., 90 mg/dL instead of 70 mg/dL) to catch lows earlier.

For detailed guidance on setting temp targets and configuring autosensitivity, consult the OpenAPS documentation.

Prepare a Sick‑Day Communication Plan

Inform your healthcare team about your OpenAPS system. Provide them with a copy of your current settings and agree on thresholds for when to call them. Also share your plan with a caregiver so they can step in if you become unable to manage alone. Write down your usual basal rates, carb ratios, correction factors, and the name of your endocrinologist. Post it where emergency personnel can see it—on your refrigerator or inside your medical ID bracelet.

Configuring OpenAPS for Active Sick Days

Once you feel symptoms coming on, take immediate action in your OpenAPS rig or phone app. The goal is to give the system more flexibility to handle rapid changes without forcing it into a corner with tight constraints.

Use Temporary Target Settings

Set a temporary target of 130–150 mg/dL (7.2–8.3 mmol/L). This tells OpenAPS to aim higher than normal, reducing the chance of insulin stacking and subsequent lows. You may also want to increase the target during the night to prevent unnoticed hypoglycemia while sleeping. If you use a night‑time profile, create a separate “sick‑night” profile with a target of 140–150 mg/dL. Some users prefer to raise the target even higher—up to 160 mg/dL—if they are vomiting or unable to eat anything carbohydrate‑containing.

Adjust Maximum IOB and Basal Limits

If you experience significant insulin resistance, you might need to increase the max IOB temporarily—but only if you can monitor every 2‑3 hours. Alternatively, reduce the max IOB if you are not eating much, to avoid overdosing. Most experienced users keep the default maximum IOB and rely on temp targets instead. If your glucose remains stubbornly high (above 250 mg/dL) for more than 4 hours despite an elevated target, consider manually increasing your basal rate by 10–20% in the pump’s settings (not in OpenAPS, which will then incorporate it).

Enable or Strengthen Autosensitivity

If your OpenAPS version supports autosensitivity (also called “autosens”), ensure it is active. This feature uses CGM data from the past 8–24 hours to detect whether you are more or less sensitive to insulin than usual. During illness, autosensitivity will detect resistance and automatically increase basal rates—exactly what you need. You can also adjust the autosens parameters: increase the autosens_max_ratio from the default 1.2 to 1.5 or even 2.0 during severe illness to allow the system to deliver more insulin when needed. Review your logs after 24 hours and revert if you see frequent lows.

Daily Management During Illness: Monitoring and Manual Overrides

Even with OpenAPS handling most adjustments, you must remain vigilant. Check your CGM at least every 15–20 minutes, and confirm with finger‑stick readings if symptoms feel off. Nausea, vomiting, and diarrhea can cause rapid dehydration and electrolyte imbalances, which interfere with insulin action. Keep a log of your glucose, ketones, and any symptoms; it will help both your care team and your own decision‑making.

Dealing with Nausea and Vomiting

If you cannot keep food down, suspend or reduce any planned meal boluses. OpenAPS will continue to use your current target and basal settings. For severe vomiting, you may need to switch to a temporary lower basal rate (or even zero basal) to prevent dangerously low glucose. However, always test for blood ketones if your glucose is above 250 mg/dL (13.9 mmol/L) and you are vomiting—this could indicate DKA requiring emergency care. If you have a pump with a “suspend” function, use it sparingly and only after you’ve confirmed that the vomiting is not due to food poisoning or a stomach bug that will resolve quickly.

Carb Correction for Hypoglycemia

When blood glucose drops below your target, treat with 15 grams of fast‑acting carbs (glucose tablets, juice, or regular soda). Because OpenAPS may already be reducing basal insulin, you might need fewer carbs than usual—start with 10–12 grams. Recheck after 15 minutes. If you have difficulty swallowing, use glucose gel that can be absorbed through the gums. Avoid high‑fat or high‑protein carbs like chocolate or milk when treating lows, because they slow down absorption.

Manual Dose Adjustments – When to Override OpenAPS

OpenAPS relies on CGM trends and your settings. If you see a pattern of near‑flat but high glucose for hours, you can administer a micro‑bolus (e.g., 0.5–1 unit) via your pump interface. The system will then incorporate that insulin into its future calculations. Do not override the system with a full correction bolus unless you have confirmed with a finger‑stick and you are certain you can handle the resulting drop. A better approach is to raise your temp target by another 10–20 mg/dL if you want aggressive reduction of highs. If you use SMB (Super Micro Boluses), make sure SMB is enabled and that you have set a reasonable max SMB value—illness is a prime time for SMB to help.

Managing Ketones During Illness

Sick days dramatically increase the risk of diabetic ketoacidosis (DKA), even when glucose is not extremely high. Test for blood ketones every 4–6 hours during illness, and immediately if your glucose exceeds 300 mg/dL (16.7 mmol/L) or if you feel nauseous, confused, or have fruity breath. Keep a ketone meter next to your bed so you can test without having to get up in the middle of the night.

Treating Trace to Moderate Ketones

  • Drink plenty of sugar‑free fluids – water, broth, or electrolyte drinks (avoid carb‑heavy sports drinks). Aim for at least 8 ounces every hour. Dehydration worsens ketosis.
  • Do not delay bolus insulin – take a small correction dose to bring down glucose. OpenAPS will auto‑bolus if you use the “eat carbs” zero‑carb trick, but you can also manually deliver a small amount (e.g., 0.5–1 unit for children, 1–2 units for adults) depending on your correction factor. Repeat every 2–3 hours if ketones persist.
  • If ketones are moderate (0.6–1.5 mmol/L) and your glucose is high, consider changing your infusion set – a site failure can cause ketones even with correct dosing. Change the set even if the old site looks good.
  • Check your insulin: if it has been exposed to heat or cold, or if you have had the same vial open for more than 28 days, replace it with a fresh vial. Degraded insulin loses potency and contributes to hyperglycemia and ketones.

When Ketones Are High (≥1.6 mmol/L)

This is a medical emergency. Contact your healthcare provider or go to the ER immediately. Do NOT rely on OpenAPS alone. High ketones require frequent insulin and intravenous fluids, which a hospital can provide. Even if OpenAPS is delivering insulin, you may need medical intervention to correct the underlying cause (infection, dehydration, site failure). Bring your pump and CGM supplies to the hospital, along with a list of your current OpenAPS settings.

For authoritative guidelines on DKA prevention, see the JDRF resource page on sick‑day management.

Adjusting OpenAPS for Specific Illnesses

Different illnesses produce distinct glucose profiles. Tailoring your approach can improve outcomes. Here are the most common scenarios and recommended adjustments.

Stomach Flu (Gastroenteritis)

Vomiting and diarrhea cause rapid loss of fluids and may make you unable to eat. In this scenario, lowering your temp target to 100–110 mg/dL can be dangerous because hypoglycemia can develop quickly. Instead, use a high temp target (140–150 mg/dL) and reduce your max IOB. Test for ketones early – often stomach flu can induce ketosis even with normal glucose. If you cannot keep down fluids, consider using an anti‑nausea suppository (if your doctor approves) or go to urgent care for IV fluids. Do not rely on OpenAPS to solve dehydration.

Common Cold or Flu

Fever and inflammation increase insulin resistance. You may need a higher target (130–140 mg/dL) and possibly a slight increase in basal rates (via temporary target adjustments). The autosens feature will typically compensate within 8–12 hours. Be careful with cough syrups – many contain sugar (up to 10–20 grams of carbs per dose). Log any carb‑containing medications as carbs in OpenAPS so it accounts for them. Some sugar‑free cough drops still contain sorbitol, which can raise glucose in some people. Use liquid formulations labeled “sugar‑free” and check the carb content on the label.

Infection with High Fever

Very high fever (>102 °F / 39 °C) can cause extreme insulin resistance. Consider setting a temporary target of 150–160 mg/dL and manually increasing max IOB by 20–30% if you are comfortable doing so. Monitor closely; fever spikes can also lead to hypoglycemia when the fever breaks (as the inflammation resolves quickly). Keep glucose tablets nearby and consider reducing your target back to 130 mg/dL once the fever drops below 101 °F. If you take fever‑reducing medications like acetaminophen (paracetamol), be aware that it can sometimes interfere with CGM readings for some sensor types – confirm with finger‑stick if you see unexpected patterns.

Urinary Tract or Sinus Infections

These often cause a gradual rise in blood glucose over several days without dramatic symptoms. Use OpenAPS’s autosens to detect the resistance trend. Set a temp target of 120–130 mg/dL early, even before you feel very sick. If your glucose remains high despite correction, consider starting a course of antibiotics sooner – early treatment reduces the metabolic burden.

Psychological and Logistical Considerations

Illness is stressful, and managing a DIY system while feeling terrible can be overwhelming. Simplify your decisions by writing down a step‑by‑step sick‑day protocol on a single card. Include: (1) set temp target 130–150, (2) enable autosens if not already, (3) test ketones, (4) hydrate, (5) call doctor if ketones > 1.5 or vomiting > 6 hours. Share this card with a family member. Consider using a phone timer to remind you to check glucose every 20 minutes. Do not beat yourself up if you have a few hours of high glucose – focus on staying hydrated and ketone‑free.

When to Seek Medical Help – Red Flags

OpenAPS is an incredibly powerful tool, but it cannot replace professional emergency care. Call your doctor or 911 if you experience any of the following:

  • Blood glucose consistently over 300 mg/dL (16.7 mmol/L) despite multiple correction boluses (and you have changed the infusion set).
  • Blood ketones over 1.5 mmol/L, especially if rising or accompanied by vomiting.
  • Severe vomiting or diarrhea lasting more than 6 hours with inability to keep down fluids – you may need IV fluids.
  • Difficulty breathing, confusion, or loss of consciousness – potential DKA or hypoglycemia emergency.
  • Signs of severe dehydration: dry mouth, sunken eyes, dark urine, or dizziness when standing.
  • Fever above 103 °F (39.4 °C) that does not respond to fever reducers within 2 hours.

Always communicate your OpenAPS usage to emergency personnel – they may not be familiar with DIY systems, so bring a simple written summary of your insulin delivery settings and contact info for your diabetes team. You can print a one‑page “medical alert” from the OpenAPS website.

Recovering After Illness – Resuming Normal Settings

Once you are feeling better for 24–36 hours (no fever, able to eat and drink normally), gradually return to your pre‑illness OpenAPS settings. Do not abruptly revert – your metabolism may still be unstable. For the first day back, keep a slightly higher target (e.g., 110–120 mg/dL) and monitor for unusual lows. Re‑enable any disabled features and review your autosens data to ensure it has returned to baseline. Check your ketone meter; you may still have trace ketones for another day. Hydrate well for the next 48 hours.

After recovery, log what worked and what didn’t in your diabetes diary. Consider sharing your experience with the OpenAPS community forums – collective knowledge makes the system better for everyone. You can also update your sick‑day protocol card based on what you learned.

Summary – A Proactive Approach to Sick‑Day Control

Using OpenAPS during illness is not a set‑it‑and‑forget‑it affair. Preparation, smart configuration adjustments, frequent monitoring, and early recognition of complications are essential. By setting temporary targets, enabling autosensitivity, stocking supplies, and knowing when to call for backup, you can keep your glucose levels safer and avoid the extremes that make sick days especially dangerous.

Remember that OpenAPS is a DIY system – you are ultimately responsible for your own management. Always consult your healthcare provider for personalized medical advice. For more detailed technical guidance on sick‑day configurations, visit the official OpenAPS documentation and the Diabetes UK sick‑day rules. For general sick‑day guidelines for people with diabetes, the CDC’s page on managing diabetes while sick provides additional support.