Lantus is a depot-type of insulin versus other insulin, like Prozinc, which is more of an "in and out" type of insulin. I'm not sure how much that means to you at this point. What that information translates to is that Lantus doesn't work best if you have to change shot times and alter doses rapidly. Lantus likes consistency! The effect of a dose is cumulative. In addition, a late shot acts like a dose reduction and an early shot acts like a dose increase and they don't cancel each other out. If you need to shoot late, it can take several days of shot time adjustments to get back on your schedule. Even with skipping, the issue is that a skipped shot may help with your schedule but it has an effect on the depot. It the depot doesn't have a chance to stabilize, you are losing the advantage of using Lantus. All of the above applies if you change the dose based on the pre-shot number which could mean you end up changing the dose at every shot (and doses should be held a minimum of 3 days for the depot to stabilize). Giving more insulin will not have an effect on how long the insulin works. What it could do is drop your cat's numbers like a rock which sets up a dangerous situation if you're not home to monitor.
Shooting late, given the above information, is obviously less of a problem than shooting early. Shooting 3 hours early could put your cat at risk for dropping into low numbers without your being there to monitor. In addition, it looks like you are basing your dose on the pre-shot numbers. Lantus dosing is based on the nadir (lowest point in the cycle) which, if your cat is fairly typical, is somewhere around +6, give or take. It sounds like you're gone a good part of the day and can't test. It's very important to do what you can to get spot checks when you're home in the evenings, weekends, holidays, etc. Just to be repetitive, Lantus dosing is based on the nadir and not the pre-shot number.
Many vets are far more familiar with the "in and out" type of insulin and will tell you that it's OK to shoot anywhere within 2 hours of your preferred shot time. This really isn't the case with Lantus. As much as I hate to suggest this, if your schedule is as unforgiving as it sounds, you might want to talk to your vet about switching to Prozinc. It is one of the insulins that is recommended by the American Animal Hospital Assn for the treatment of feline diabetes (Lantus is the other insulin) and you can slide the dose based on pre-shot numbers. You will still need to try to get more test data so you have some idea how low the dose is taking Bumpsy and if a dose reduction is indicated. If your schedule is crazy because of the holidays and will calm down in a week or so, Lantus may be fine providing you start getting more tests. If what you've described is typical, you may want to consider Prozinc.
