But now, when the nation desperately needs more coronavirus tests to get a handle on the virus’s spread, this efficient approach has become worthless in many places, in part because there are simply too many cases to catch.

Pooled testing only works when the vast majority of batches test negative. If the proportion of positives is too high, more pools come up positive — requiring each individual sample to then be retested, wasting precious chemicals.

Nebraska’s state public health laboratory, for example, was a pooling trailblazer when it began combining five samples a test in mid-March, cutting the number of necessary tests by about half.

But the lab was forced to halt its streak on April 27, when local positivity rates — the proportion of tests that turn up positive — surged past 10 percent. With that many positives, there was little benefit in pooling.