This is a very big question as measuring endotoxin directly in the human blood to me is very challenging.. The current methods of measuring endotoxin are based on the responses of the immune defense of some vertebrates and invertebrates to endotoxin. Endotoxins being pyrogens elicit release of pro-inflammatory cytokines in vertebrates and eventually results in pyrexia. One could think that monitoring of cytokines( such as IL-1, IL-6 and TNF -alpha) levels in humans cold give a clue as to the presence and levels of endotoxin in the blood. However, other pathogen-associated- molecular patterns such as lipotechoic acid, proteoglycans, beta 1-3 glucans are foreign substances that may also cause the release of such cytokines. The challenge is going to be ascertaining that the cytokines levels are solely due to the presence and levels of endotoxin in the blood.

In my opinion, some immunocompromised patients (especially in children), is programmed by overproduction of interleukins and tumor necrosis factor alpha, which does not adequately assess the level of endotoxemia. That is, the production of proinflammatory factors will be many times greater than the amount of endotoxin in the blood. Theoretically, the most accurate way to early diagnosis of the etiology of the infection process, because at the moment known pathogenicity factors in the majority of microorganisms. This will try to detect endotoxin in the blood of specific chemiluminescence method.

I can add to previous advices.  In our experiments, and investigations of other scientists preferred commercially proven LAL test. For example, one of the links.

http://www.ncbi.nlm.nih.gov/pmc/articles/pmid/19755625/

But most of all we are interested in processes in the body after administration of endotoxin. We are talking about the level of proinflammatory cytokines in the blood.

I enclose a link to such a method in one of our articles.

http://www.jphysiol.org/cgi/pmidlookup?view=long&pmid=14565987

All the best.

After a lot of experiments, we gave up on LAL.

Here is our solution, but the absorber is difficult to get.

http://www.google.com/patents/DE10247430A1?cl=en

Nowadays there are several commercial kits for endotoxin detection apart from LAL.

- Endolisa (Hyglos) is based on the rFC, recombinant factor C to detect the presence of LPS. But this method has not been accepted by the FDA, it can only be used for research.

- Pyrogent (Lonza) based also in the detection of rFC. Again not approved by the FDA.

- Hek-blue (Invivogen) that uses TLR4 (culture of cell lines is needed)

- PyroDetect (Merck and Biotech) that quantifies the LPS regarding to the IL-beta

Anyway, if you need to detect endotoxin in a lab environment (not for diagnosis) I would suggest  to use a biosensor based system with an specific and sensitive bioreceptor (aptamer, peptide).

Article New Antiseptic Peptides To Protect against Endotoxin-Mediated Shock

Article Implementation and Characterization of a Fully Miniaturized ...

Article Determination of endotoxin through an aptamer-based impedanc...

The trick with endotoxin measurement in blood is that the concentration of (more or less unspecified) neutralizing factors of LPS varies from individual to individual. We measured LPS in blood for years by adding 3-4 known increasing LPS amounts to each blood sample and calculated the LPS concentration from the intercept with the x-axis, using a chromogenic assay. The nice thing about this method is that you hereby can also measure the endotoxin-neutralizing capcity of the blood, which might be an underestimated indicator of subliclinical chronic inflammation. The most detailed description you can find here: http://www.ncbi.nlm.nih.gov/pubmed/2050995. Although a rather old publication, we have applied this method for about two decades in our lab and it worked reliably: http://www.ncbi.nlm.nih.gov/pubmed/9394106, http://www.ncbi.nlm.nih.gov/pubmed/9394106, and http://www.ncbi.nlm.nih.gov/pubmed/10845660. Cheers, Alex

phenol-sulphuric acid is an approved method for LPS measurement?

Since there are endotoxin inhibitors in blood it should be better to measure the reactivity of endotoxin instead of the antigen concentration. Such a chromogenic FAa generation test is similar to a thrombin generation test.