The link between blue baby syndrome and nitrates in drinking water is widely accepted, but some studies indicate that other contaminants, or dietary nitrate sources, may also play a role in the syndrome. Diagnosis
When diagnosing blue baby syndrome, it is important to perform a thorough history and physical exam. When obtaining the history, it is important to determine the timing of symptoms and to ask about risk factors/exposures, such as prenatal history or access to well-water. On physical exam it is important to visualize where the cyanosis is present to differentiate between peripheral and central cyanosis. Central cyanosis is typically visible as a blueish discoloration over the entire body and mucous membranes. In contrast, peripheral cyanosis typically has a blueish discoloration over the extremities. Cyanosis can be noted in babies around the lips, tongue, and sublingual area, where the skin is thinnest. In addition, it is important to observe the infant for signs of respiratory distress, visualized as nasal flaring, subcostal retractions, etc. Examination should include a respiratory and cardiac assessment.One of the key tools in diagnosing is a pulse oximeter to determine oxygen saturation. While severe cyanosis can be easily noticed, an oxygen saturation as low as 80% causes only mild clinical cyanosis that is difficult to see. Additionally an arterial blood gas is useful, for example in the case of methemoglobinemia the PO2 can be expected to be normal even with a low oxygen saturation.
Conditions in which there is poor blood flow to the systemic circulation, such as coarctation of the aorta suggests that the body does not receive the oxygenated blood it requires with resultant cyanosis.The five most common cyanotic heart defects that may result in Blue Baby Syndrome include the following:

Methemoglobinemia
Methemoglobinemia which can be acquired or congenital, occurs when the iron in hemoglobin is oxidized from Fe2+ to Fe3+, leading to poor binding of oxygen. Additionally, the oxygen that is already bound is held more tightly to the hemoglobin due to a higher affinity, resulting in less oxygen delivery. A methemoglobin level >1.5g/dL causes cyanosis. The most common congenital cause is a deficiency in the enzyme cytochrome b5 reductase which reduces methemoglobin in the blood. However, in infants the most common cause of methemoglobinemia is acquired through the ingestion of nitrates through well water or foods. Infants younger than 4 months are at greater risk given that they drink more water per body weight, they have a lower NADH- cytochrome b5 reductase activity, and they have a higher level of fetal hemoglobin which converts more easily to methemoglobin. Additionally, infants are at an increased risk after an episode of gastroenteritis due to the production of nitrites by bacteria. The sources of nitrate can include fertilizers used in agricultural lands, waste dumps or pit latrines. For example, nitrate levels are subject to monitoring to comply with drinking water quality standards in the United States and other countries.