LDT stands for laboratory developed test, a diagnostic test that is designed, built, and used entirely within a single laboratory rather than being sold as a commercial kit. These tests are used across medicine to detect diseases, guide treatment decisions, and screen for genetic conditions. They play an especially large role in cancer care, rare disease diagnosis, and infectious disease testing, where commercial options may not yet exist or may not meet a specific clinical need.
How LDTs Differ From Commercial Tests
Most diagnostic tests you encounter in healthcare are commercially manufactured products, packaged and sold to laboratories around the country. These are called in vitro diagnostics (IVDs), and they go through a standardized review process before reaching the market. An LDT, by contrast, never leaves the building. A single laboratory designs the test, validates it internally, and runs patient samples on-site. The same lab that created the test is the only one that uses it.
This distinction matters because it affects how the test is regulated, how its accuracy is verified, and how quickly it can reach patients. Commercial test kits must typically receive clearance or approval from the FDA before being sold. LDTs have historically operated under a different, lighter set of rules.
Where LDTs Are Used
LDTs fill gaps that commercial tests cannot. They are common in oncology, where laboratories develop custom genetic panels to detect specific cancer mutations, monitor how well a treatment is working, or identify hereditary cancer syndromes in patients and their relatives. Circulating tumor DNA tests, which look for fragments of cancer DNA floating in the bloodstream, are a growing category. These tests can help diagnose certain cancers, guide treatment planning, and detect whether a cancer has returned after treatment.
Most genetic and genomic tests are LDTs. When a patient gets tested for a rare genetic condition, the test was likely developed in-house by a specialized reference laboratory. During infectious disease outbreaks, LDTs also serve a critical role because laboratories can design and deploy new tests faster than commercial manufacturers can bring a packaged product to market. The early months of the COVID-19 pandemic highlighted both the speed advantage and the quality concerns that come with this flexibility.
How LDTs Are Regulated
Two federal systems govern diagnostic testing in the United States, and LDTs have long occupied an unusual space between them. The Centers for Medicare and Medicaid Services (CMS) regulates clinical laboratories through the Clinical Laboratory Improvement Amendments (CLIA), a set of rules that requires labs to validate test performance before using it on patients. CLIA certification is mandatory, and laboratories performing LDTs must meet the standards for high-complexity testing.
The FDA, on the other hand, regulates medical devices, including diagnostic tests. The agency has maintained that it has legal authority over all in vitro diagnostics, LDTs included, under the Medical Device Amendments of 1976. But for decades, the FDA chose not to exercise that authority as “a matter of general practice.” This hands-off approach meant that LDTs reached patients without the independent analysis and verification that commercially distributed tests undergo.
CLIA requires laboratories to establish performance specifications for their tests, including accuracy, precision, and the range of results the test can reliably detect. What CLIA does not require is the kind of independent, pre-market review the FDA applies to commercial diagnostics. This gap has been at the center of a long-running policy debate.
Safety Concerns
The lighter oversight of LDTs has raised real patient safety questions. Many LDTs have not been fully vetted for sufficient analytical and clinical validity through existing regulatory pathways. Analytical validity refers to whether the test accurately measures what it claims to measure. Clinical validity refers to whether the result actually corresponds to a meaningful medical condition or outcome. Both matter, and gaps in either can cause harm.
A false negative result means a disease goes undetected, delaying life-saving treatment while the condition progresses. A false positive triggers unnecessary and sometimes invasive treatment, additional costs, significant anxiety, and delays in reaching the correct diagnosis. Diagnostic errors, including those from unreliable tests, contribute to an estimated 40,000 to 80,000 preventable deaths per year in the United States. The risk is highest in rural areas and under-resourced health systems, and it becomes especially dangerous during infectious disease outbreaks or bioterrorism threats, where inaccurate results can cascade into public health failures.
The 2024 FDA Rule Change
On April 29, 2024, the FDA announced a final rule that changes the regulatory landscape for LDTs. The rule amends FDA regulations to make explicit that in vitro diagnostics are medical devices under federal law, even when the manufacturer is a laboratory. In practical terms, this means the FDA is ending its decades-long practice of looking the other way on LDTs.
The agency outlined a four-year phaseout of its hands-off enforcement approach, organized in five stages. In the first year after the rule takes effect, laboratories will need to comply with requirements for reporting adverse events and any corrections or removals of their tests. By the end of the second year, labs must register their tests and meet additional requirements. High-risk tests face the end of enforcement discretion no earlier than October 2027. All remaining low-risk tests must comply by the four-year mark, no earlier than April 2028.
After the phaseout is complete, the FDA generally expects LDTs to meet the same requirements as commercially distributed diagnostics. This includes pre-market review, quality system standards, adverse event reporting, establishment registration, device listing, and labeling requirements. The agency has carved out targeted exceptions for certain categories of laboratory-manufactured tests, but the broad direction is toward consistent oversight regardless of where a test is made.
Why LDTs Still Matter
Despite the safety concerns, LDTs serve an essential function in modern medicine. They allow laboratories to develop tests for conditions where no commercial option exists, to customize panels for specific patient populations, and to innovate faster than the traditional product-approval pipeline permits. In oncology alone, the rapid evolution of genetic testing has fundamentally changed how cancers are diagnosed, classified, and treated. Much of that progress has been driven by laboratory-developed tests that could be designed and deployed without waiting years for commercial clearance.
The tension between speed and safety is at the heart of the LDT debate. Stricter regulation may improve test accuracy and patient outcomes, but it could also slow the introduction of new tests, particularly for rare diseases and emerging infections where the patient population is small and commercial development is not economically attractive. The four-year phase-in timeline reflects an attempt to balance these competing priorities, giving laboratories time to adapt while gradually closing the regulatory gap that has existed for decades.