Lactic Acid Stinging Test

CD Formulation provides professional Lactic Acid Sting Test (LAST) to help our clients to test the efficacy of cosmetic products in all aspects. The Lactic Acid Sting Test is considered to be the most effective way to evaluate sensitive skin. The mechanism of the test is that when the skin barrier is damaged, lactic acid enters the skin and stimulates the unmyelinated class C nerves, resulting in a tingling sensation.

Why Need Lactic Acid Sting Test?

Stinging tests have been used as a method of assessing nerve sensitivity in the skin. Tingling appears to be a variant of pain that develops rapidly and disappears quickly whenever the appropriate sensory nerves are stimulated. The test depends on the tingling sensation caused by the chemicals used on the nasolabial folds. Sensitive skin refers to a state of hyperreactivity of the skin that occurs under physiological or pathological conditions. The main clinical manifestations are the tendency of the skin to show subjective symptoms such as burning, tingling, itching, and tightness when stimulated by physical, chemical, or psychiatric factors, with or without objective signs such as erythema, scaling, and capillary dilatation.

To perform the lactic acid stinging test, an aqueous lactic acid solution was wiped on the test site with a cotton swab, while an inert control substance was used to the contralateral test site. After application, a moderate to severe tingling sensation occurs in the "tingling group" after a few minutes. Subjects were then asked to describe the intensity of the sensation using a score scale.

Subjective Evaluation

First, respondents were asked to self-assess their skin's sensitivity based on their subjective symptoms such as burning, stinging, itching and tightness when stimulated by triggers, and to draw their own conclusions on whether they have sensitive skin or not.

Semi-Subjective Evaluation

Semi-subjective evaluation is done through chemical probe (irritant) tests, including the lactic acid stinging test, capsaicin test, chloroform-methanol mixture test, SLS skin irritation test, and dimethyl sulfoxide test. Among them, lactic acid stinging test (LAST) is one of the more widely used evaluation methods. When the skin barrier is damaged and lactic acid enters the skin, it stimulates the unmyelinated class C nerves, thus producing a tingling sensation, which is simple, effective and reproducible, and has been widely used in evaluating soothing and repairing type of test studies.

Objective Evaluation

Objective assessment is the evaluation of small changes in the skin through the measurement of biophysical parameters using non-invasive techniques with the aid of biomedical engineering.

Lactic Acid Sting Test

Lactic acid solution was applied to the nasolabial folds and either cheek under constant temperature and humidity, and the subjects were asked about their conscious symptoms at 2.5 and 5 minutes, respectively, and then scored, with a score greater than a certain amount defined as a positive lactic acid sting response.

For the duration of the test, skin repair is a long-term process, so it is necessary to do a long-term efficacy test, usually for four weeks. The lactic acid sting test requires the use of two samples per day for 28 days. The test and evaluation is done at D0, D14 and D28 respectively. If the lactic acid sting score value is lower and statistically significant after 28 days of using the test product compared to the value before use, the skin is less reactive to external stimuli, i.e. the product has a repairing effect.