Lactic Acid Stinging Test

Protheragen-ING provides a comprehensive Lactic Acid Sting Test (LAST) service to evaluate the repair efficacy of cosmetic products against potential stinging or irritation caused by lactic acid, helping customers evaluate and optimize product formulations to minimize discomfort and improve customer satisfaction.

Learn About the LAST

Lactic acid is an alpha hydroxy acid commonly used in skin care products for its exfoliating and skin-renewing properties. However, it may cause discomfort, stinging, or irritation, especially in people with sensitive or damaged skin.

LAST is a widely used method for screening sensitive skin. It also provides valuable insights into evaluating the efficacy of cosmetic cosmetics.

Raw material for skin care product.

Protheragen-ING's LAST Service

Our testing service is a semi-subjective evaluation method. The test mechanism is that when the skin barrier is damaged, lactic acid enters the skin and stimulates the unmyelinated C nerves, resulting in a tingling sensation.

Our service steps are as follows.

  1. Plan customization - We design experimental plans according to your specific requirements.
  2. Subject Selection - We select test subjects, typically individuals with different skin types and sensitivities, to participate in the study.
  3. Patch Application - We perform patch testing to evaluate potential stinging or irritation from lactic acid. Usually on the forearm or back, a small area of skin is marked and treated with a lactic acid solution.
  4. Observation Period - The plaque is left undisturbed for a specified period of time. During this time, subjects were asked to pay attention to any sensations such as tingling, burning, itching, or redness.
  5. Subject Feedback - After the observation period, subjects were asked to provide feedback about their experience with each patch.
  6. Grading and Analysis - We utilize a standardized grading system to quantify the intensity of sting or stimulation experienced by test subjects.
  7. Results and Recommendations - We offer to prepare a comprehensive report summarizing the methodology, test results and observations. The report may include a rating of stinging or irritation, statistical analysis, and recommendations for formula adjustments if necessary.

Specific Test Methods

In a constant temperature and humidity environment, 50 μL of 10% lactic acid solution was applied to the nasolabial fold and one cheek. The subjects were asked about their subjective symptoms 2.5 minutes and 5 minutes after application and scored on a 4-point scale.

The two scores are added together, and those with a total score of no less than 3 are considered to have a positive lactic acid sting reaction.

The LAST score before and after using the test sample can directly reflect its repair effect on skin barrier function. As for the test time, skin repair is a long-term process, so long-term efficacy testing is required, and the general time is four weeks.

Subjects need to use the sample twice a day for 28 consecutive days. Test and evaluate at D0, D14, and D28 respectively. If after using the test product for 28 days, the LAST score is lower than before use, and it is statistically significant, it means that the skin's reactivity to external stimuli is reduced, that is, the product has a repair effect.

Skin repair effect.

Please feel free to contact us to discuss your specific testing needs, inquire about pricing, or learn more about our turnaround times. We will respond within 2-4 working days.


Daily: 9.30 AM–6.00 PM
Sunday : 9.30 AM–1.00 PM
Holidays: Closed

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