The condom split last week, or your last test is so long ago you can't remember the date. Now you're sitting there working out whether to order a home test or book an appointment.

The honest answer is that both work well. Just not for the same situation. If you have no symptoms and you're testing as a matter of routine, a home test will usually serve you better than waiting three weeks for a slot. If you've had burning pain when you pee for days, a clinic will do more for you than a checkout page.

So which one fits where you are right now?

First: not all home tests are the same

Two quite different things go by that name, often called self-tests online. The difference decides how much your result is worth.

The rapid test you do at home. You take a drop of blood yourself, put it on a cassette and read the result after about 15 minutes. HIV tests like this are sold over the counter in Germany. They're convenient, but they need more distance from the exposure than a lab test does. That distance is called the diagnostic window: the time that has to pass before an infection can be detected at all. Rapid tests only pick up antibodies, which your immune system has to produce first, so the wait is longer.

The kit with lab analysis. You collect the samples at home, so urine and a little blood, and send everything to a lab. The same method runs there as after a blood draw in a clinic. The only difference is who takes the sample. Reality Check works this way and also includes swabs for the throat and rectum, which most other home tests leave out.

When we talk about home tests below, we mostly mean the second kind.

What a home test is good at

You can start now. No appointment, no waiting list, no question of whether a practice is taking new patients. In bigger cities, slots at dermatology or gynaecology practices are often booked out weeks ahead.

You don't have to talk to anyone. That sounds trivial, but it's the most common reason people put testing off. The prospect of sitting in a waiting room and then explaining exactly what you did and with whom keeps more people from testing than the cost does. Take that step away and people test more often, and testing more often means finding infections earlier.

You test the sites that usually get missed. Chlamydia and gonorrhoea can settle in the throat and the rectum too, including when you've never had anal sex. An infection at those sites almost never shows up in urine, because a urine test reflects the urethra. It only gets found if a swab is taken there as well. Reality Check includes throat and rectal swabs as standard. In a clinic you often have to raise them yourself, and in real life that happens less often than it should.

You know the price up front. When you're paying for tests yourself in a clinic, the cost sometimes only becomes clear afterwards.

Where a home test runs out of road

You collect the sample. For urine and a finger-prick that's straightforward. For swabs, quality depends on how carefully you follow the instructions. Studies show self-collected swabs perform almost as well as clinician-collected ones, provided the instructions actually get read. That's the part that occasionally goes wrong.

Nobody looks at anything. A lab test detects pathogens. It doesn't detect an ulcer, a rash, a wart or a swelling. Some sexually transmitted infections are diagnosed by eye, and a home test can't do that.

There are no follow-up questions. In a conversation it often emerges that something else should be tested for too. A kit tests what's in the box.

A test can't treat you. The result on its own doesn't finish the job. With gonorrhoea you'll need a prescription, from a clinic or an online consultation. With HIV or syphilis there's a confirmatory test first, then ongoing care at an HIV specialist service or a dermatology clinic. In Germany, public health offices and Aidshilfe checkpoints cover that step free of charge and anonymously. With chlamydia we take that step off your hands: if your Reality Check result is positive, treatment comes straight from us, with no further appointment.

What a clinic is good at

Symptoms belong there. Discharge, burning, pain, blisters, sores, itching, a rash, pelvic pain or testicular pain: these need an examination and usually fairly prompt treatment. Even with a fast lab, a few days pass between sending the sample and getting the result. Those are days in which an infection can keep travelling further up.

Some things have to happen in person. A speculum examination, an internal exam, a proper look at your skin. None of that can be posted.

Diagnosis and treatment in one place. With a bacterial infection you can walk out with the prescription. Plus the advice around it: who you need to tell, when you're in the clear again, whether a test of cure is needed.

After a high-risk exposure, hours matter. If you think you may have been exposed to HIV in the last 72 hours, the thing to ask about is HIV PEP, emergency treatment that can still prevent infection. You get it at an HIV specialist service or the A&E department of a larger hospital, and the sooner the better. Within 24 hours is considerably more effective than on day three. A test is no help at that point, because a recent exposure wouldn't show up yet.

Some of it is covered. In Germany, women get one free chlamydia urine test a year on statutory insurance up to their 25th birthday. In pregnancy it's covered at any age, along with the other tests in antenatal care. Tests are generally covered when you have symptoms too. Without symptoms and without a clinical indication you'll usually be paying yourself. Public health offices and Aidshilfe checkpoints also offer testing that is cheaper and often anonymous.

Where a clinic is annoying

You need an appointment first. With acute symptoms you'll usually get seen somewhere. For a routine check without symptoms you're often looking at several weeks, and that's exactly the stretch in which the good intention tends to quietly disappear.

It's uncomfortable. For a lot of people it stays that way, however professional everyone involved is.

You don't automatically get everything you need. Ask for an STI test and you may well get a urine test and nothing else. If you have sex involving your throat or rectum, you have to raise those swabs explicitly.

Judgemental questions still happen. Not everywhere, but often enough to be a real factor. One bad experience and the next test gets postponed.

Which route, when

A home test fits when

• you have no symptoms
• you're testing as routine, say every three to six months with changing partners
• a new relationship is starting and you both want to test
• the exposure was a while ago and the diagnostic window has passed
• you'd rather stay anonymous

A clinic fits when

• you have symptoms, can see changes or are in pain
• the exposure was less than 72 hours ago, in which case go now
• you're pregnant or trying to be
• a partner has tested positive
• a home test came back positive
• you're not sure what you should be testing for in the first place

The timing matters more than the route

Whichever you pick, testing too early is wasted. Every pathogen needs time before it can be detected.

• chlamydia and gonorrhoea: detectable from about seven days, reliably after 10 to 14
• syphilis: around six weeks
• HIV, lab test: six weeks after the last sexual contact, at which point infection can be ruled out
• hepatitis C: twelve weeks, because it's an antibody test
• HIV, rapid test at home: longer than the lab test, so twelve weeks

For most of what a test panel covers, six weeks since the last contact is enough. A negative result three days afterwards tells you very little.

If you have symptoms, you don't wait out any window. You go to a GP, dermatology, gynaecology or urology practice. What's happening then is something other than a routine test: you may have symptoms of an infection that needs treating.

If a home test comes back positive

First, the reassuring part: for everyone who deals with this professionally a positive result is routine, and in the vast majority of cases it's very treatable. HIV and syphilis get a confirmatory lab test first, bacterial infections go straight to treatment.

That's also where most home tests stop. You get your result and organise the rest yourself: find an appointment, present the result, get a prescription, explain everything again. With chlamydia, Reality Check works differently. If the result is positive, treatment comes straight from us, with no further appointment and no waiting. For the other pathogens you'll still need a clinic, but at least you'll walk in knowing exactly what you're dealing with.

Telling partners is part of it too. Awkward, and it stops an infection bouncing back and forth between you and possibly other people.

If you test regularly

If your partners change often, a fixed routine beats occasional one-offs. Every three to six months is the usual recommendation, more often if you have a lot of partners. It's worth sorting out your HIV protection in the same breath: PrEP is the most effective option where exposure is higher, and in Germany it's covered by statutory insurance if you meet the criteria. Anyone on PrEP tests regularly anyway, that's part of the care.

Where exposure is very high, antibiotic PEP can also be discussed. On current evidence it's an option for men who have sex with men and for trans women. It's prescription-only, a clinician reviews it first and it doesn't replace testing.

Not an either/or

Most people end up needing both over time, the home test and the clinic. The home test covers everything routine, and it lowers the barrier at exactly the point where most people drop out. A clinic covers everything that needs examining, looking at or treating quickly.