How the COVID-19 testing lab at St. Paul's Hospital increased capacity when supplies ran low

·2 min read

In mid-August, the laboratory doing COVID-19 testing at St. Paul's hospital in Vancouver was facing a problem: the reagent required to carry out the tests was in short supply.

"There was a lack of availability," said Dr. Daniel Holmes, director of pathology and laboratory medicine at the hospital. "Without the reagent — the chemicals that go into it — it's like having a car without gasoline."

So Holmes and his team at the lab turned to a trick used in virology; they began working on a way to pool test samples together.

The idea was to combine the samples from four patients — a number determined by the positivity rate they were finding at the St. Paul's lab, between three and seven per cent.

"If you have a whole bunch of samples and most of them test negative for a disease, you can mix all of the samples together, and if the mix tests negative, then you can infer that all of the samples that went into the mix must be negative," explained Holmes.

The idea was simple enough, but the task of automating the process with a robotic machine and computer code took a while.

Holmes said it wasn't until Sept. 20 that the system was ready for its first live run — just as the second wave of the pandemic began to ramp up.

"The robot ... scans all the barcodes, it tells the server which specimens are in which well, and in the end, it reports out all the negatives," he said.

The four samples in the pools that test positive have to then be tested individually, so if the positivity rate increases, the method becomes inefficient.

Holmes said he was getting anxious as positivity rates climbed in recent weeks, but so far they've been able to continue mixing samples at St. Paul's.

The technique has eased the workload on staff at the lab, as well as getting four times as many tests out of the reagent used for the pooled samples, said Holmes, noting that the lab typically does about 40 per cent of its daily tests — which range from 1,000 to 1,700 per day — using the pooling technique.

As well as effectively reducing the required resources, mixing samples hastens the time it takes to process most patients' tests, though Holmes said there's a three-hour wait if a pool needs to be tested again as individual samples.

"If they are one of the people who's fortunate enough to have a negative test, their result is going to come back to them, somewhere between three and 10 hours earlier," he said.

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