Directlabs uses LabCorp which is one of the two main lab companies in the US so the result would be the same as if you saw your local health care practitioner who uses LabCorp for labs.

At least for 25(OH0)D, LabCorp is a significantly better choice than Quest. For other lab tests, I've not heard any specifics.

I can't speak to accuracy or applicability of ALCAT but wrt to ELISA it will depend on what you're testing.

http://en.wikipedia.org/wiki/ELISA

Because the ELISA can be performed to evaluate either the presence of antigen or the presence of antibody in a sample, it is a useful tool for determining serum antibody concentrations (such as with the HIV test[3] or West Nile Virus). It has also found applications in the food industry in detecting potential food allergens such as milk, peanuts, walnuts, almonds, and eggs.[4] ELISA can also be used in toxicology as a rapid presumptive screen for certain classes of drugs.

I assume your're talking food allergies or intolerances?

Using ELISA for this purpose is very controversial. This is an excellent overview of some of the concerns that come up regarding ELISA testing for food intolerances: http://www.betterhealthusa.com/public/282.cfm

Having said that, the following is quite interesting: http://cep.sagepub.com/content/30/7/829.short?rss=1&ssource=mfr

Diet restriction in migraine, based on IgG against foods: A clinical double-blind, randomised, cross-over trial

Abstract Introduction It is well-known that specific foods trigger migraine attacks in some patients. We aimed to investigate the effect of diet restriction, based on IgG antibodies against food antigens on the course of migraine attacks in this randomised, double blind, cross-over, headache-diary based trial on 30 patients diagnosed with migraine without aura. Methods: Following a 6-week baseline, IgG antibodies against 266 food antigens were detected by ELISA. Then, the patients were randomised to a 6-week diet either excluding or including specific foods with raised IgG antibodies, individually. Following a 2-week diet-free interval after the first diet period, the same patients were given the opposite 6-week diet (provocation diet following elimination diet or vice versa). Patients and their physicians were blinded to IgG test results and the type of diet (provocation or elimination). Primary parameters were number of headache days and migraine attack count. Of 30 patients, 28 were female and 2 were male, aged 19–52 years (mean, 35 ± 10 years). Results: The average count of reactions with abnormally high titre was 24 ± 11 against 266 foods. Compared to baseline, there was a statistically significant reduction in the number of headache days (from 10.5 ± 4.4 to 7.5 ± 3.7; P 

Unlike IgE antibodies, there simply isn't much known about what IgG antibodies actually mean but the above study is interesting and hopefully another group will pick up the work and start replicating.