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Miscellaneous diagnostic tests 

Miscellaneous diagnostic tests
Chapter:
Miscellaneous diagnostic tests
Author(s):

Stephen Chapman

, Grace Robinson

, John Stradling

, Sophie West

, and John Wrightson

DOI:
10.1093/med/9780198703860.003.0066
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date: 24 September 2021

Skin prick tests

These may be useful in identifying specific allergens causing immediate hypersensitivity (IgE-mediated) reactions. They may influence management and guide allergen avoidance. They are also used to help define the presence of atopy. Triggers for contact urticaria, atopic eczema, and suspected food allergy may also be identified. The results are available almost immediately (compared with 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 manage adverse reactions.

The allergens tested should be identified from the history and may include common aeroallergens, e.g. pollens (grass, tree, weeds), moulds (Alternaria alternata, Aspergillus fumigatus, Cladosporium, Penicillium chrysogenum), house dust mite (Dermatophagoides pteronyssinus), and animal dander (dog epithelium, cat pelt).

Practical points

  • Testing should be performed off antihistamines (7 days) and omalizumab (≥6 months). Tricyclic antidepressants and phenothiazines may also block response. Oral steroids appear not to suppress test results

  • Very small risk of anaphylaxis; adrenaline and resuscitation equipment should be available. Particular care is needed with food and latex testing

  • Clean the skin with 70% alcohol solution. 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

  • Prick the skin through the allergen drop, using a needle (do not draw blood). This should be with a calibrated lancet (1mm), held vertically, or a hypodermic needle held at 45° to the skin

  • The positive control is usually histamine and the negative control the diluent (usually glycerinated saline)

  • Read the histamine control after 10min and the allergen extracts after 15–20min. A positive result is an itchy weal, which should be compared with the controls, as some subjects react to the skin prick alone (dermatographism)

  • Different test solutions are standardized to give a mean weal diameter of 6mm across sensitive subjects

  • A weal of 3mm or more is considered positive (indicating 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 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 are no RCT data to show that this method can identify atopic from non-atopic individuals, as identified from skin prick tests.

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 bronchoconstriction

  • Nebulized hypertonic (2.7–5%) saline is administered via a face mask. Afterwards the patient expectorates sputum into a sterile pot

  • If transmission of infection (e.g. TB) is likely, perform the test in a negative pressure room, with appropriate protection of staff and other patients. Do not perform on the open ward or outpatient department

  • Send sputum promptly to microbiology for staining and culture and direct immunofluorescent testing for Pneumocystis jirovecii (if indicated). Sputum for differential cell counts is mixed with 0.1% dithiothreitol, diluted with saline, and then filtered and centrifuged.

Bronchoprovocation testing

Several techniques can be used to assess bronchial hyperreactivity, including pharmacological challenges (non-specific bronchoprovocation testing), exercise challenge (for exercise-induced asthma), food additive challenge, and antigen challenge.

Non-specific bronchoprovocation testing

  • Helpful if there is diagnostic doubt regarding the diagnosis of asthma

  • Typically use inhaled methacholine or mannitol. Methacholine directly stimulates smooth muscle contraction. Mannitol works indirectly via release of endogenous mediators and is more specific but less sensitive

  • Should be performed by experienced personnel, with facilities to deal with acute bronchospasm

  • Patients need to omit regular asthma therapy (inhaled/oral steroids: 2–3 weeks; antihistamines: 3 days; tiotropium: 1 week; LABA: 2 days)

  • Increasing doses of provocation agent are given sequentially, with the FEV1 measured after each dose

  • Methacholine Given in a nebulizer using solutions ranging from 0.03mg/mL to 16mg/mL. 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 a 20% fall is known as the PC20 and may lie between the concentrations of the last two doses. Asthma is indicated by a PC20 ≤8mg/mL. Normal subjects have a PC20 >16mg/mL. Intermediate responses may be seen in patients with family history of asthma or in those with COPD or CF or when recovering from viral infections

  • Mannitol Proprietary dry powder inhaler (Aridol®) used. Provocation dose causing 15% fall in FEV1 (PD15) is noted. Asthma indicated by PD15 at cumulative dose of ≤635mg.