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Ovid: Oxford Handbook of Respiratory Medicine

Authors: Chapman, Stephen; Robinson, Grace; Stradling, John; West, Sophie Title: Oxford Handbook of Respiratory Medicine, 1st Edition Copyright ©2005 Oxford University Press > Table of Contents > Part 4 – Practical procedures > Chapter 70 – Some diagnostic tests Chapter 70 Some diagnostic tests P.700
Skin prick tests These may be useful in identifying specific allergens causing immediate hypersensitivity reactions. They may influence management and guide allergen avoidance. They are also used to help define atopy. Triggers in contact urticaria, atopic eczema, and suspected food allergy may also be identified. The results are available almost immediately (compared to a RAST test for specific IgE), and correlate well with RAST test results. They should be carried out by staff trained to read the tests and deal with adverse reactions. The allergens tested should be identified from the history, and usually include common aeroallergens e.g. grass, house dust mite (D. pteronyssinus), and cat dander. Practical points

  • Testing should be performed off steroids and anti-histamines
  • There is a very small risk of anaphylaxis; adrenaline and resuscitation equipment should be available. Particular care is needed with food and latex testing
  • Put a drop of allergen on the skin (usually the inside forearm). A range of allergens are available commercially. Fresh produce should be used for suspected fruit and vegetable hypersensitivity
  • Lance the skin through the allergen drop using a needle (do not draw blood). This should be with a calibrated lancet (1 mm), held vertically, or a hypodermic needle held at 45° to the skin
  • The positive control is usually histamine, and the negative control the dilutent (usually saline)
  • Read the test after 15 minutes
  • A positive result is an itchy weal, which should be compared with the controls, as some subjects react to the skin prick alone (dermatographism)
  • Test solutions are standardized to give a mean weal diameter of 6 mm
  • A weal of 3 mm or more is considered positive (indicating allergen sensitization)
  • A positive result does not prove that the clinical symptoms are due to bronchial hyperresponsiveness to the tested allergen, but do raise clinical suspicion. Positive results can occur in those without symptoms, and false negatives do occur.

RAST or radioallergosorbent blood tests are more specific, but less sensitive and more expensive than skin prick tests, but give similar information. There is no risk of anaphylaxis and the patient does not need to stop steroids or antihistamines for the test to be performed. Unconventional tests Electrodermal allergy testing (using a Vegatest machine) was developed as an aid to homeopathic prescribing, and is widely used in complementary medicine to assess allergic status to food and environmental allergens. It is based on small changes in skin electrical impedance at acupuncture points, in response to allergens placed in an electrical circuit. There is no RCT data to show that this method can identify atopic from non-atopic individuals, as identified from skin prick tests. P.701
Technique of induced sputum

  • Used to investigate for infection (e.g. TB, PCP) or airway inflammation
  • Patients rinse their mouth and clean their teeth to minimize oral contamination. Give inhaled salbutamol, to minimize bronchconstriction
  • Nebulized hypertonic (2.7–5%) saline is administered via a face mask. Afterwards the patient expectorates sputum into a sterile pot
  • If infection is likely, perform test in a negative pressure room, with appropriate protection of staff and other patients. Do not perform on the open ward or out-patient department
  • Send sputum promptly to microbiology for staining and culture, and direct immunofluorescent testing for PCP (if indicated). Sputum for cell counts is mixed with 0.1% dithiothreitol and diluted with saline and then filtered and centrifuged.

Methacholine challenge testing

  • Methacholine induces bronchospasm in people with hyperreactive airways. Helpful if there is diagnostic doubt regarding the diagnosis of asthma
  • Should be performed by experienced personnel, with facilities to deal with acute bronchospasm
  • Increasing nebulized doses of methacholine are given systematically, with the FEV1 measured after each dose
  • If there is a 20% fall in FEV1, or if the highest dose of methacholine has been given, the test is stopped
  • The concentration of drug causing the 20% fall is known as the PC20
  • Asthma is indicated by a PC20 of below 8 mg/ml. Normal subjects have a PC20 > 16 mg/ml.

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