Negative Test Results
A negative result simply means that no eggs were detected at the time the test was taken, it doesn't mean that there are no pinworm present in your horse.
If you still suspect a problem you may want to test again 2-4 weeks later, keeping an eye out for other symptoms in the meantime such as tail rubbing, beansprout type worms in droppings or signs of the sticky substance the eggs are laid in around the anus which can be a yellow/orange colour. Wiping the bottom daily with baby wipes can be a good preventative way of removing eggs if you suspect a problem.
Positive Test Results
If pinworm is confirmed, either with a sellotape test or by seeing them in the horse's droppings, treat with one of the more old fashioned wormers like pyrantel (Strongid P) or fenbendazole (Panacur 5 day guard) which are more effective at treating pinworm than ivermectin or moxidectin because of the way these are metabolised in the horse's gut.
Using neem oil on the horse's skin is also sometimes beneficial to prevent the eggs sticking once they're laid. Couple this with good stable hygiene and disinfect any areas where the horse may rub his tail to help reinfection.
Occasionally we may see pinworm eggs in worm egg count samples but this is not common because the eggs are generally laid outside the horse around the anus area. Nevertheless they will be reported in your result if they are present in the sample and above treatment advice should be followed.
You can check again using another pinworm test 2 weeks after wormer to check the efficacy.