The short answer
A patch test means trying a new product on a small spot before you use it on your face or body. Put a small dab on the inside of your forearm, leave it uncovered, and repeat twice a day for three days. If the spot stays calm, you can go ahead. It takes almost no effort and it can spare you a lot of misery.
Why it happens
Skin can react to something new even when the label looks harmless. The ingredient list tells you what is in the jar. It cannot tell you how your own skin will respond. Two people can use the same product, and one never notices a thing while the other ends up with red, stinging, bumpy skin.
Reactions do not always show up at once. Some appear within minutes. Others take a day or more to build, which is why one quick dab and a glance is not much of a test. Skin on the face and neck is thinner and more reactive than skin on the arm, so a reaction there is both more likely and more upsetting. And the skin that has reacted before, which is what we mean by sensitive, tends to react again.
A reaction on a small patch of forearm is easy to live with. The same reaction across your face is a different week. That is the whole idea.
Many people skip the test because it feels like a lot of fuss for a jar of moisturizer. It is one dab, twice a day, for three days. Most of the effort is remembering to look at the spot. A habit of testing also means that when something does bother you later, you can narrow it down much faster.
What helps
Wash the inside of your forearm with plain water and a gentle cleanser, and dry it well. Put a small dab of the new product on the skin, about the size of a pea or smaller, and rub it in lightly. Leave it uncovered. Do not cover it with a bandage, and do not wash it off right away.
Do this twice a day, morning and evening, for three days. Use the same spot each time. Keep going even if the first day looks fine, since some reactions take a while to show. During the test, try not to put other new products on that same spot, or you will not know which one to blame.
Check the spot each day in good light. You are looking for redness, stinging, bumps, itch or swelling. A brief warm feeling from rubbing is not the same as stinging that builds. If you are not sure, compare the spot with the skin next to it.
If you see redness, or feel stinging or bumps, wash the spot off with lukewarm water and stop. Do not try the product on your face to see whether it behaves differently. Give the skin time to settle.
Test one new product at a time. If you try three at once and something happens, you have no way to know which was responsible. Read the ingredient list too, and check it for anything you already know bothers you. Test again if the formula changes, or if it has been a long time since you last used something, because skin changes over the years.
Pick a spot that will not rub against clothing or a watch strap, since friction can redden skin by itself and confuse the result. The inner forearm works well because the skin there is thin and easy to see, and you will not be tempted to wash it off in the shower without thinking.
A passed test is encouraging, not a promise. Reactions can still develop later. If your skin starts to bother you after weeks of use, stop the product, even if it passed the test.
For a young child, use a tiny dab on the back of the leg or the inner forearm, and watch the skin and the child's face, because they may not tell you when something stings. Check with your child's doctor if you are unsure.
When to ask for help
Some reactions are an emergency. If your lips, eyes, tongue or face begin to swell, or if you have trouble breathing, get help right away. Call 911 or go to the nearest emergency room. Do not wait to see whether it passes. This is rare, and it is the reason a patch test is worth doing.
For a smaller reaction, wash it off, stop the product, and see how the skin does over the next day or two. If it does not settle, or if it spreads, blisters or weeps, call your doctor or ask your pharmacist. Bring the product with you so they can read the label. If you remember what the spot looked like, or can take a photo, that helps too.
The AHOTO note
Golden contains oats and soy, and we recommend a patch test before you use it. Glowabe contains both as well. Pure Shea is one ingredient, cold pressed unrefined East African shea butter, so there is only one thing to test. Test it all the same.