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What to do if…
you receive a lab result marked critical or urgently abnormal without explanation

Short answer

A “critical/urgent” flag means you should speak to a clinician as soon as you can (the same day where possible). If you feel seriously unwell right now, call 999 or go to A&E instead of waiting for a message.

Do not do these things

  • Do not ignore it because you “feel fine” — some critical results need action before symptoms appear.
  • Do not stop, start, double, or skip prescribed medicines (especially insulin, diuretics, lithium, anticoagulants, heart medicines) unless a clinician tells you to.
  • Do not try to “fix the number” with supplements, fasting, extra fluids, or restrictive diets — that can make things worse.
  • Do not assume it’s definitely yours or definitely correct; mix-ups and lab issues can happen.
  • Do not rely on online forms or non-urgent messages alone when it’s marked critical — use a phone call.

What to do now

  1. Check it’s definitely your result. Confirm your name and date of birth on the report, and that the test date/time and requesting service match what you had done.
  2. Write down the essentials (so you can say them quickly). Test name, value, units, the “critical/urgent” flag, reference range, and when it was taken.
  3. Check for “go now” symptoms. If you have chest pain, severe shortness of breath, fainting/collapse, new confusion, seizure, signs of stroke, uncontrolled bleeding, a severe allergic reaction, or you feel rapidly worsening: call 999 or go to A&E now.
  4. Phone the clinician/service that ordered the test (first choice).
    • If it was your GP, phone the surgery and say: “I’ve received a result flagged critical/urgent with no explanation. I need a same-day clinician call-back / duty doctor review.”
    • If it was a hospital clinic, phone the clinic line/secretary and ask for the on-call team (or advice line) for a result flagged critical/urgent.
  5. If it’s out of hours or you can’t reach the ordering service: use NHS 111 and clearly state it’s a lab result flagged critical/urgent, the test/value, and whether you have symptoms.
    • If the patient is a child under 5, use 111 by phone (not 111 online).
  6. If you’re asked to repeat the test, ask the “today” questions. “Do I need to avoid food/meds before repeating? Where should I go today? What symptoms mean I should go to A&E?”
  7. Flag high-risk context immediately. Pregnancy; diabetes; kidney disease; heart rhythm history; chemotherapy/immunosuppression; recent severe vomiting/diarrhoea; or medicines commonly tied to urgent lab issues (for example insulin, diuretics, lithium, anticoagulants).

What can wait

  • You do not need to interpret the result, diagnose yourself, or search for worst-case scenarios right now.
  • You do not need to change long-term medication, supplements, diet, or lifestyle before a clinician advises you.
  • You do not need to request full records or debate “normal ranges” before you have a same-day plan.
  • You do not need to contact multiple services at once; use one clear route (ordering clinician/team → NHS 111 if unreachable → A&E/999 if unwell).

Important reassurance

Seeing “critical” on a screen is frightening, especially without context. The flag is a safety signal that a clinician should connect the number to your symptoms, history, and medicines. Sometimes results appear in records after review, but you can still receive a worrying-looking flag without an immediate explanation — it’s appropriate to ask for urgent clarification.

Scope note

These are first steps only, to reduce risk and get timely clinical review. After review, the next step may be repeat testing, same-day treatment, medication changes, or monitoring depending on the specific test and your situation.

Important note

This guide is general information, not medical diagnosis or a substitute for clinical care. If you are seriously unwell, rapidly worsening, or unsure you can safely wait, use urgent services (999/A&E). If you cannot reach your usual clinician and the result is flagged critical/urgent, NHS 111 can help route you to the right level of care.

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