Every applicant filing Form I-693 has to clear a tuberculosis screening, but not every applicant gets the same test. Ask two people in the same family what happened at their civil surgeon’s office and you may get two different answers: one describes a blood draw, the other describes a small injection under the skin and a return visit two or three days later. The difference usually comes down to a single variable — age — and understanding why can save families a confusing trip back to the clinic.
USCIS TB screening on the immigration medical exam exists to catch two things: people who have been exposed to Mycobacterium tuberculosis and, through follow-up testing, people whose exposure has progressed into contagious disease. Neither the skin test nor the blood test tells a civil surgeon which of those two situations an applicant is in on its own — both only signal exposure. But which of the two initial tests gets ordered in the first place is where age comes in.
The Age Rule Behind IGRA vs. the Skin Test
For most applicants two years of age and older, civil surgeons default to an Interferon-Gamma Release Assay, or IGRA — a single blood draw processed in a laboratory. The older Tuberculin Skin Test (TST), sometimes called a PPD test after the purified protein derivative injected under the skin, is generally reserved as an option for healthy children younger than five. That narrower age band exists because young children’s immune systems can produce less reliable IGRA results, and the skin test has a longer track record in that age group specifically.
In practice, this means a parent filing alongside a toddler may walk out of the same appointment having had two entirely different procedures: the parent gets a blood draw that resolves in one visit, while the child gets a skin test that requires a second appointment 48 to 72 hours later so the civil surgeon can measure the reaction at the injection site. Applicants who assume every family member goes through an identical process are often the ones surprised by the extra follow-up visit.
Why the Blood Draw Became the Default for Almost Everyone Else
The shift toward IGRA as the standard test for applicants outside that young-child window reflects a few practical advantages that matter specifically in an immigration-exam setting. A blood draw needs only one appointment — there is no second visit to read a skin reaction, which matters for applicants juggling a tight filing timeline or traveling from out of town for their exam. IGRA results also are not thrown off by a prior BCG vaccination, which many applicants received as children in countries where BCG is still standard; a skin test on that same person can produce a reaction that looks positive even without real TB exposure, requiring extra interpretation. For a civil surgeon signing a federal form, a test that is harder to second-guess is the easier one to rely on.
Comparing the Two Tests
| Factor | Tuberculin Skin Test (TST/PPD) | IGRA Blood Test |
|---|---|---|
| Typical age use on Form I-693 | Healthy children under 5 | Applicants 2 years and older, the general default |
| Number of visits needed | Two — injection, then a reading 48-72 hours later | One — a single blood draw |
| Affected by prior BCG vaccination | Yes, can cause a false-positive reading | No |
| What a positive result confirms | Exposure to TB bacteria, not whether it is active | Exposure to TB bacteria, not whether it is active |
| Next step after any positive result | Chest X-ray required | Chest X-ray required |
A Positive Result Means the Same Thing Either Way
Whichever test an applicant takes, a positive result triggers the identical next step: a chest X-ray to check for signs of active disease, since neither the skin test nor the blood test can distinguish inactive exposure from contagious illness by itself. That follow-up process — what the X-ray shows and what happens next — is its own subject; the point relevant here is that the choice between IGRA and the skin test only affects how you get screened, not what a positive screen means once you have one.
What This Means for Booking Your Appointment
If your household includes both adults and young children preparing I-693 exams together, ask the civil surgeon’s office in advance which test each family member will receive and plan your calendar around it. An applicant getting only IGRA can often complete their TB screening portion in a single visit. A family with a child under five should expect to schedule a return trip for the skin-test reading before the civil surgeon can finalize that portion of the exam. Knowing this ahead of time — rather than discovering it mid-appointment — is the difference between a filing timeline that stays on track and one that picks up an unplanned week of delay.
It also helps to ask what happens if the child ages into the adult testing window between now and a future filing. A civil surgeon practice that regularly handles family cases can usually tell you on the phone which test a given applicant will need before you ever sit down in the exam room, simply based on the birthdate on file. That single question — skin test or blood draw — is worth confirming before the appointment rather than after, since it determines whether your visit ends that day or continues into a second one.
None of this changes what USCIS is ultimately screening for. The age-based choice between IGRA and the skin test is a matter of which tool a civil surgeon uses to detect TB exposure, not a different standard of exposure or a different bar for admissibility. Two applicants — one tested with a blood draw, one tested with a skin reaction read three days later — are being held to the identical requirement. The only thing age changes is the path they take to get there, and for a growing share of applicants, that path now runs through a single blood draw rather than a return trip to the clinic.