Additionally, a false negative result is possible with a blood test if the person has recently been taking certain drugs, such as antibiotics or proton-pump inhibitors.The diagnosis of Helicobacter pylori can be made by:

Urea breath test (noninvasive and does not require EGD);
Direct culture from an EGD biopsy specimen; this is difficult and can be expensive. Most labs are not set up to perform H. pylori cultures;
Direct detection of urease activity in a biopsy specimen by rapid urease test;
Measurement of antibody levels in the blood (does not require EGD). It is still somewhat controversial whether a positive antibody without EGD is enough to warrant eradication therapy;
Stool antigen test;
Histological examination and staining of an EGD biopsy.The breath test uses radioactive carbon to detect H. pylori. To perform this exam, the person is asked to drink a tasteless liquid that contains the carbon as part of the substance that the bacteria breaks down. After an hour, the person is asked to blow into a sealed bag. If the person is infected with H. pylori, the breath sample will contain radioactive carbon dioxide. This test provides the advantage of being able to monitor the response to treatment used to kill the bacteria. The possibility of other causes of ulcers, notably malignancy (gastric cancer), needs to be kept in mind. This is especially true in ulcers of the greater (large) curvature of the stomach; most are also a consequence of chronic H. pylori infection.
It has been suggested that all cases of cancrum oris develop from pre-existing NUG, but this is not confirmed. Furthermore, the vast majority of cases of NUG and NUP will not progress to the more severe forms, even without treatment. References
J Lindhe, NP Lang, T Karring (editors) (2008) "Clinical periodontology and implant dentistry" 5th edition, Blackwell Munksgaard, pp. 413,459 
MG Newman, HH. Takei, PR Klokkevold, FA Carranza (editors) (2012) "Carranzas clinical periodontology" 11th edition, Elsevier/Saunders, p. 165