Being told that your genetic test (NGS, or next-generation sequencing) 'failed,' that the result held nothing meaningful, or that the tissue has to be sampled again can be deflating. Weeks or months slip by, and the back-and-forth trips to the hospital add up. Knowing why this happens can help you understand where things are stuck right now and prepare for the next step.
NGS looks broadly, all at once, at what genetic changes exist inside a tumor. If a particular variant is found, a matching targeted therapy or clinical trial may become an option; conversely, learning that a specific variant is absent is also useful information for choosing a direction. The goal of the test is essentially to draw a 'treatment map,' which is why waiting on the result feels so frustrating.
There are two main ways to get the material for the test. One is a tissue biopsy, in which a piece of the tumor is removed and examined directly. The other is a liquid biopsy, which analyzes tumor-derived DNA circulating in the blood (ctDNA, or circulating tumor DNA). A liquid biopsy is helpful when it is hard to reach the tumor with a needle, since a few tubes of blood can be enough — but 'just drawing blood' does not guarantee a full answer.
There are reasons a liquid biopsy can come back 'empty' or thin. The amount of tumor DNA shed into the blood varies a great deal with the tumor's size and activity. When tumor burden is low — and especially when chemotherapy is working well and the tumor is shrinking — the tumor DNA in the blood drops, and important variants can fall below the test's detection limit. Even when metastases look prominent on imaging, that does not always match the amount of DNA in the blood, so the expectation that 'a tumor this size should surely show up in blood' can miss.
This is why a repeat test using tissue may be suggested. But there are conditions here too. NGS needs enough tumor cells of adequate quality to analyze. Requirements vary by lab, but many ask for several unstained slides (often around 10 to 20) that also contain tumor cells above a certain proportion (tumor content). Tissue taken with a fine needle, older archived samples, or tissue with few tumor cells and a lot of necrosis or fibrosis may not yield enough slides or enough tumor content to run the test. In other words, 'a biopsy was done' and 'enough tissue was secured for NGS' are not the same thing.
A few things are worth arranging in advance. Before a tissue procedure, clearly tell the team performing it that the sample is intended for NGS genetic testing, and check ahead of time how many slides and what tumor-content threshold the ordering lab requires — this can spare a wasted trip. You can also ask your doctor whether an existing paraffin block or slides stored at another hospital could be used, and whether a liquid or tissue biopsy fits your situation better right now. Knowing that a test failing on the first attempt is not unusual can make it a little easier to accept that a failure or a redo may be about the nature of the sample, not anyone's fault.
This article is for general information and does not replace the diagnosis or care of an individual patient. Testing methods, whether to repeat a test, and plans for obtaining tissue differ from person to person, so please decide together with your own medical team.