Having your teeth cleaned is not usually thought of as a procedure that needs preparation. It can bleed, though, and a dentist who takes that seriously is doing something sensible.
Scaling removes hardened deposits from below the gum line. It is often described as a deep clean, and the part of it that matters for this question is that it may be uncomfortable, it can cause bleeding, and it is not entirely free of risk. The gum gets small amounts of trauma, and if the gums were already inflamed and bleeding, the trauma is more obvious.
That makes scaling comparable to a minor surgical procedure, and the preparation for it follows a similar logic.
What is actually being tested
"Blood test" is a general term covering dozens of possible investigations. The point is not to test everything. It is to establish a small number of things before a procedure that involves bleeding.
A full blood count. This is the baseline. It gives red cell count and haemoglobin, which indicates anaemia; white cell count and differential, which can point to a current infection somewhere in the body; and platelet count, which matters for clotting.
Clotting function. Scaling can cause bleeding. In a healthy person, that bleeding usually stops on its own within minutes. Where clotting is impaired, it may not. Finding that out beforehand rather than mid-procedure is the point.
This is also where anticoagulant and antiplatelet medication becomes relevant. People taking warfarin, direct oral anticoagulants, aspirin or clopidogrel need to have this discussed explicitly before a procedure. The decision to stop or adjust anything is a medical one made with a doctor, not a decision to make on the basis of a dental article.
Liver and kidney function, and blood sugar. These reflect general health and affect healing. Uncontrolled diabetes and impaired liver function can both make periodontal disease harder to control and can affect clotting and recovery. These are particularly worth checking in older patients and those with known medical conditions.
Screening for blood-borne infections. Hepatitis B, hepatitis C and HIV are blood-borne infections. Syphilis may also be included in some screening panels because it can be transmitted through blood or other routes, can be present without symptoms, and may affect clinical management. Screening requirements vary by country and clinic.
The part that is not really about paperwork
There is a second reason, and it is the one people rarely hear stated plainly.
Dental clinics are shared clinical spaces. Instruments that contact one patient's mouth are reprocessed and sterilised before use in another patient, and blood and saliva can spray. A clinician who knows in advance what is on a patient's blood test results or infection status can take appropriate precautions.
It also helps protect everyone else in the room. When a patient has an active infection, or an infection that has not yet been diagnosed, knowing about it allows the team to use appropriate precautions in a space where aerosols are generated.
Being clear about what this is not: it is not a judgement, and it is not a refusal to treat someone because of a diagnosis. A clinician who knows in advance adjusts the procedure, uses precautions, and simply works more carefully. The genuinely worse situation is a clinician working without the information.
There is a practical reality worth acknowledging too. A full screening panel before every routine clean is not universally required, and where a patient has a recent test result, a clear medical history and an examination, the residual risk may be small. Patients should feel free to ask what is being checked and why, and to ask about the ones that are not routine.
What to do if you are asked
Ask what is being tested and what the result would change. If you have had a recent blood test, bring the results: they may cover part of what is needed.
If you take anticoagulants, antiplatelets or have a bleeding disorder, say so before the appointment, not during it. It changes the plan, and it is the single most useful thing you can tell the dentist.
If you are otherwise well and have had routine bloods recently, this is usually straightforward. The tests are quick, some results come back within hours to a day, and infection screening may take longer. Where everything is normal, nothing about the appointment changes.