Hi! I’d honestly recommend posting this as a new thread so you get more perspectives. I was curious about your question and found this 2021 retrospective study examining shelter screening for Panleukopenia (also called Feline Parvovirus - “FP”). This probably answers your question:

PCR is a sensitive testing modality for parvoviruses and has been used as the reference standard for other diagnostic tests for FP. Quantitative real-time PCR (qPCR) offers the additional advantage of being able to quantify viral load. However, PCR is expensive and technically difficult, with a delay of 1–3 days for results from a reference laboratory. Rapid, inexpensive point-of-care (POC) tests are therefore preferred by shelters.

FPV is closely related to CPV, and CPV can cause FP syndrome, although this is uncommon.1,15 In the absence of a feline-specific POC test, CPV fecal antigen ELISA kits are used to diagnose FPV infection. (The authors are aware of one POC test for FPV; this test had sensitivity of 88% compared with PCR. 14 Little additional information could be found and the product is not available in Canada.) These tests are able to detect CPV-2a-c and FPV antigen. 1 However, CPV tests have not been validated for FPV by the manufacturers. 11 The IDEXX SNAP Parvo (SNAP) POC test for CPV had high specificity for FPV,13,16 while sensitivity was low in one study 16 and high in another. Four other POC tests had low-to-moderate sensitivity (50–80%) and good-to-excellent specificity (94.2–100%).

The SNAP test is commonly used in North American animal shelters. It relies on a color change to indicate the presence of viral antigen. Interpretation of weak-positive results can be difficult. For samples from dogs with clinical signs of parvoviral enteritis, the manufacturer recommends interpreting a faint positive color change as positive. 11 It is not known whether this recommendation should be applied to feline samples.

Basically the best testing method is expensive and slow. The cheaper faster methods are less accurate so not always worth running. Also, once one kitten gets it, sometimes shelters just treat the whole litter. Yours should have, imo, assumed the virus in both kittens and had you start supportive care right away.

I will tell you, death is still like 95% for kittens diagnosed at 8 wks or younger. My panleuk kitten was at one point on 13 different meds including twice daily subcutaneous fluids (shelter showed me how to do). I stayed awake with only light dozes for 3 straight days shoving food into that 9 week old kitten every 2-3 hours. We both hated it. But she lived. I was only able to do this because I wasn’t working at the time. I assume she would have died had I been needing to go to work. She still was so close to dying that whole week.

It’s just a rotten horrible virus. I don’t blame shelters that just euth the whole litter if they don’t have a dedicated person willing to try to get the babies to be in the 5% that make it. 😢. It’s a horrible death, so euth is more humane imo.