We call your body’s ability to fight infection ‘host defence’. The host defence investigation is a series of tests we run to identify the cause of a lung problem and the best method of treatment.

The investigation consists of several different tests.

Lung function tests

Lung function tests measures how well your lungs are working. You will be asked to breathe into a number of instruments, which measure different aspects of your lung function.

Our staff will give you full instructions on how to perform each test. You may also have to give a blood sample from your finger or earlobe to measure oxygen levels.

Ciliary function tests

These tests assess the function of the body’s cilia. Cilia are small hair-like structures that constantly beat to clear mucus from the airways. In some people these are immotile, beat too slowly or move in an uncoordinated fashion. Any of these conditions will predispose you to chest infections.

Nitric oxide test

These show how well the cilia – the small hairs that beat constantly to clear mucus from the airways – are working. The first test we will carry out simple involves blowing into a machine.

If this test suggests a problem, we will use a special brush to take a sample of your nose cilia for further investigation.

Nasal muccociliary clearance (NMCC) test/saccharine test

This test measures your levels of nitric oxide. You simply blow into an instrument. Patients who have cilia that are not working properly have low levels of nitric oxide.

But if you have high levels of nitric oxide, this may suggest inflammation in the lung.

Shuttle walking test

This test measures your fitness levels. We will ask you to walk a ten metre course, marked by two cones. During the test you will hear a series of bleeps – we will ask you to finish the course before the next bleep sounds.

The bleeps become more frequent over time to encourage you to increase your effort. The test ends when you are unable to keep up with the bleeps, when you become tired or become breathless.

You will need to wear comfortable clothing and footwear for this test.

St George’s respiratory questionnaire (SGRQ)

We use this questionnaire to find out about your current health condition. It asks you questions about your illness and the effect it has on your daily life.

There are no right or wrong answers with this, we want you to be honest and let us know what is happening.

Blood tests

We will often perform blood tests in the outpatients’ department before your stay in hospital. We may ask you to have further blood tests during your stay.

Analysing your blood shows us how vulnerable you are to respiratory infections. It also lets us measure levels of inflammation in the body.

Skin prick test

We test for several common allergies, including animal hair and dust mites. We will put a drop of the substance being tested on your arm and then prick the surface of the skin through the drop.

We then watch for any reaction (usually a temporary reddening of the skin).

Sputum (phlegm) collection

We will ask you for several phlegm samples during your stay. One of these will be a sample of all the phlegm you produce in a 24hour period.

We will give you sterile containers for this and will use the samples to check the amount, colour and consistency of the phlegm you are producing.

Sweat test

This is a screening test for cystic fibrosis. This is an inherited condition which causes some of the body’s glands to produce thicker and stickier mucus than normal, affecting the lungs and digestive system.

We may perform a sweat test to check that your symptoms are not caused by cystic fibrosis. A sweat test is a simple, painless procedure in which we measure the amount of salt in your sweat. People with cystic fibrosis have more salt in their sweat than those without. This is why the sweat test can be used to diagnose the condition.

The process

  1. We will put two gel discs on your forearm and pass a very low electric current through them (the current does not cause any harm, it simply encourages sweating)
  2. After six minutes, we remove the pads and place a collecting duct onto your arm. The ducts will need to stay in place for 30 to 60 minutes before we remove them.

Some patients may need a second test called a nasal potential difference. If this is needed, we will explain this to you. This test is not painful or strenuous.

Chest and sinus radiography

We may take X-ray pictures of your chest and sinuses so we can examine them more closely.

Computed tomography (CT) scan

CT scan is a special X-ray which allows us to see the structure of your lungs in slices. You will lie on a bed which moves as you are being scanned.  

During the scan you will be able to talk to our radiographer via a microphone. The scans take a few minutes to appear on the radiographer screen, so you should expect to be in the CT scanner for 20 to 40 minutes.


We may ask you to participate in physiotherapy, which could be a very important part of your future lung management.

Our staff will teach you how to clear secretions from your lungs effectively.

There are currently no related conditions associated with this test.

Further questions

If you have any further questions, please contact Lorraine Ozerovitch, clinical nurse specialist – respiratory infection

Host defence unit
Royal Brompton Hospital
Sydney Street
London SW3 6NP
Tel: 020 7352 8121, and request 'bleep 1005'

Useful information

Download the host defence workup patient information sheet (PDF, 28KB)