LTT versus epicutaneous test?

To detect type IV sensitisation, 2 independent test methods are available: the LTT and the epicutaneous test. For some questions, these complement each other well (e.g. in occupational dermatology).
The key difference between the two methods is that the epicutaneous test is only validated to detect contact allergies, i.e. for allergies where the sensitisation occurred by skin contact and which manifest on the skin.
Whenever allergens are absorbed via mucous membranes, this is referred to as systemic sensitisation. Here, the LTT offers significant advantages with regard to sensitivity. Because of this, the LTT was included as a test for the detection of sensitisation to medications in the allergologic guidelines and classed as "unreservedly recommendable” by the Guidelines Committee of the Robert Koch Institute.
Even though the dermatological professional associations have until today refused to acknowledge it, it is a fact that food, mould and restorative materials for dental work are absorbed through the mucous membranes and not through the skin.
Since the sensitivity of the epicutaneous test, depending on the test allergen, has only a sensitivity of less than 70%, a negative epicutaneous test result does no rule out a sensitisation.
The committee for “Methods and Quality Assurances“ at the Robert Koch Institute also reached this conclusion in its opinion on the LTT (Bundesgesundheitsblatt 2008; 51: 1070-1076) where it recommends, based on “socioeconomic reasons“, for metals and acrylates to “....first perform an epicutaneous test and then, in case of a negative result and continued clinical suspicion, an LTT ...“. This means that the LTT is regarded as the more expensive but ultimately more sensitive method.
Today, the LTT’s sensitivity is reported as approx. 90%, subject to the test allergen.
The specificity depends on the validation of each individual allergen and the quality of the performance of the LTT. At a DIN 15189-accredited laboratory, these sources of error can largely be excluded by internal standardisation.
The advantages and disadvantages of the two test methods were highlighted in the opinion of the German Professional Association of Physicians specialised in Environmental Medicine (see page 32-36). This opinion published in 2006 in the Journal of Laboratory Methods 30: 101-6 is still current.
Therefore, in case of preventative testing preference should always be given to the LTT, because even if the epicutaneous test triggers a sensitisation of the patient, this cannot be recognised in the corresponding epicutaneous test. It would take a second epicutaneous test performed 10-14 days later to detect this sensitisation. However, this “testing” is rather performed by the dentist or orthopaedic surgeon when they introduce the replacement material into the body.
Carcinogenic and toxic substances should never be administered to a patient’s skin. Therefore, they cannot be used with the epicutaneous test. Here, the LTT as an in-vitro method should be preferred.

The committee for “Methods and Quality Assurances“ at the Robert Koch Institute summarises the advantages of the LTT as follows:
  1. Unlike the epicutaneous test, the LTT does not expose the patient to risks as no induction of sensitisation or worsening of clinical symptoms can occur with this in vitro method.
  2. The LTT results are not influenced by the condition of the skin.
  3. With the LTT, the risk of false positive results can be minimised by “careful establishment and validation“.

At the IMD-Berlin, the LTT was accredited according to DIN 15189 as early as schon 2004 and has since then been audited by the Accredited bodies (DAkkS) every 2 years. The requirements for laboratory accreditation according to DIN 15189 significantly exceed those for ISO certification, because here expert reviewers also evaluate the indications, the establishment, the validation, the conduction of ring trials, and the result interpretation of an analytical method.