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Testing for cystic fibrosis

Sweat test for cystic fibrosis: how it is done and what results mean

The sweat test measures the amount of chloride, a salt, in a small sample of sweat. People with cystic fibrosis (CF) usually have high levels, so the test is the standard way to confirm or rule out the condition after a newborn screening result or when symptoms suggest it. This guide explains who is tested, what the procedure involves, whether it hurts and how the results are read.

Last checked: 2026-10-10

Key facts

At a glance.

  • What it measures

    Great Ormond Street Hospital says a sweat test involves collecting a small volume of sweat and measuring the amount of salts (chloride), because people with CF have a higher amount of salt in their sweat than usual.

  • After newborn screening

    The NHS says if the newborn blood spot test suggests a baby may have CF, they will have another test to check the level of salt in their sweat.

  • Not painful

    Great Ormond Street says it does not hurt, although some children report a tingling sensation where the sweat is collected, and a parent can stay throughout.

  • A heel prick can be negative and a test still needed

    Great Ormond Street says a sweat test may still be needed if the newborn screening heel prick blood test was negative for CF.

  • Takes time

    Great Ormond Street says the whole procedure usually takes between 1.5 and 2 hours, with sweat collection lasting 30 minutes on each arm or leg.

What the sweat test is for

Cystic fibrosis is a genetic condition that affects the lungs and the digestive system, as Great Ormond Street Hospital describes it. One of its features is sweat that is saltier than normal. The NHS lists very salty sweat, which can leave small crystals on the skin, among the symptoms, alongside persistent coughing, frequent chest infections, large greasy poos and poor growth in children.

The sweat test is the test that measures this saltiness. The NHS says older children and adults who might have CF will have a blood test followed by a sweat test, and that babies with a suggestive newborn screening result have a sweat test as the next step.

Who is tested

Great Ormond Street lists babies or children who have had a CF suspected result on the newborn bloodspot screening test, who get many chest infections, who have frequent and unexplained loose pale poo, or who have problems gaining weight or growing properly. It says it is important to diagnose CF as soon as possible so that treatment can start.

The NHS says CF is usually found shortly after birth as part of the newborn screening programme, but if undiagnosed, symptoms may only become noticeable when a child is a teenager. Gloucestershire Hospitals says the test is also rarely performed in adults.

What happens during the test

Great Ormond Street describes the steps. A small area of skin on the arm or leg is cleaned with water, and special pads soaked in pilocarpine, a chemical that stimulates sweating, are placed on it. A small painless electric current is passed through the pads from a battery box for five minutes, then the pads are removed and the skin is washed and dried. A sweat collector is placed over the area for 30 minutes and then sent to the laboratory. The process is repeated on the other arm or leg.

During collection the child can play and eat normally but should avoid salty foods such as crisps to reduce the risk of contamination. A member of staff from the biochemistry laboratory, experienced with babies and children, does the test.

Does it hurt, and are there risks?

Great Ormond Street says the test does not hurt, though some children describe a tingling sensation. Afterwards there is a red mark where the pilocarpine stimulated the skin, which is normal and usually fades within about 24 hours.

It lists a small risk of minor skin burns and a small risk of sensitivity to the pilocarpine, which can redden the skin and sometimes raise blister-like welts. These are often mistaken for burns but are a reaction to the chemical and usually disappear within 2 to 3 hours.

Occasionally not enough sweat is collected and the test has to be repeated. The leaflet says this is a technical matter and not a result in itself.

How the results are read

Great Ormond Street says that if the chloride level is in the normal range CF is unlikely, and if it is above the normal range the referring doctor will explain what the results mean. A blood test for the genes that cause CF may also be needed. It says results can be available the following day for inpatients or children known to the CF team, but may take up to a week to reach a local consultant or GP.

Gloucestershire Hospitals publishes its interpretation, based on national guidance, which is summarised below. Other laboratories use the same national guidance, but the exact wording on your report may differ.

Sweat chloride interpretation as published by Gloucestershire Hospitals
AgeResultMeaning
Under 6 monthsBelow 30 mmol/LCF unlikely, but genetic and clinical correlation needed
Under 6 months30 to 60 mmol/LIntermediate; further assessment such as a repeat test
Over 6 monthsBelow 40 mmol/LCF unlikely, but genetic and clinical correlation needed
Over 6 months40 to 60 mmol/LIntermediate; further assessment such as a repeat test
Any ageAbove 60 mmol/LSupports the diagnosis of CF

What happens next

A borderline result is not a diagnosis. Great Ormond Street says sometimes, even with enough sweat, the result can be hard to interpret and the sweat test has to be repeated. The NHS says carrier testing is available if a relative has CF or is a known carrier, and that you can ask for it if your partner has CF or is a known carrier.

If CF is confirmed, care is provided by a specialist CF team. The Cystic Fibrosis Trust, mentioned by Great Ormond Street, offers information and resources for families.

Frequently asked

Questions people ask.

Quick, plain-English answers to the questions we hear most.

  • What does a sweat test show?

    It measures chloride, a salt, in sweat. Great Ormond Street says people with CF have a higher amount of salt in their sweat than usual.

  • Does the sweat test hurt?

    Great Ormond Street says no, although some children report a tingling sensation where the sweat is collected. You can stay with your child.

  • How long does it take?

    Great Ormond Street says the whole procedure usually takes between 1.5 and 2 hours because both arms or legs are tested, with 30 minutes of collection each.

  • Can a baby have the test if the heel prick was negative?

    Yes. Great Ormond Street says a sweat test may still be needed if the newborn heel prick blood test was negative for CF.

  • When will I get the result?

    Great Ormond Street says the next day if the child is an inpatient or known to the CF team, and up to a week if sent to a local consultant or GP. Gloucestershire says results are normally available within 3 working days.

  • What if the result is borderline?

    Gloucestershire says intermediate results need further CF assessment, such as a repeat test or further investigations.

  • Does a high result mean cystic fibrosis?

    A level above 60 mmol/L supports the diagnosis, according to Gloucestershire, but your doctor will combine it with symptoms and genetic tests.

  • Can my child eat during the test?

    Great Ormond Street says they can play and eat normally during collection but should avoid salty foods such as crisps.

  • Do adults have sweat tests?

    The NHS says older children and adults who might have CF have a blood test followed by a sweat test. Gloucestershire says it is rarely done in adults at its hospital.

Where this comes from

Sources and checking.

This guide is for information, not medical advice.

Your GP or consultant knows your history and can tell you which parts apply to you. If your symptoms are severe, rapidly worsening, or come with red-flag features, seek urgent medical care. Figures and availability change, so treat them as a guide and confirm with the provider.

Published 2026-10-10, next review 2027-10-10.

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