In the presence of the diagnostic triad of cutaneous, nervous and alimentary tract manifestations there is little difficulty in diagnosis but when the skin lesions are absent or only slightly developed the difficulty is great. One of the most important points in diagnosis is a history of preceding attacks.
There is no reliable laboratory test and the reports as to positive reactions following injections of maize extracts seem unreliable. Again there do not seem to be any antibodies in the serum of pellagrins which can be utilized in serological diagnosis. A primary requirement would be a suitable antigen. Competent workers have been unable to find any bacterial organism in the blood of pellagrins.
Erythema multiforme and dermatitis venenata seem to be the skin diseases most liable to cause confusion.
In old people with arterio-sclerotic changes and consequent mental symptoms there may be lesions of the hands or feet of more or less gangrenous type, which may be a real source of confusion. The lack of sharp delimitation of such lesions and the absence of the pellagrous stomatitis should differentiate.
Poison ivy dermatitis, if bilateral, may be confusing, as may also chapping of the hands.
In Italy a disease due to eating ergot-diseased rye meal and called ergotism may be a source of confusion as this disease shows gangrenous manifestations. The gangrene of ergotism is a dry one.
Sprue does not show the dermatitis, and the nervous manifestations are solely those of irritability or possibly slight neurasthenia. The sprue stool is not found in pellagra. See Diagnosis under Sprue.
Typhoid pellagra may be confused with severe typhoid fever or other acute infectious diseases or with conditions associated with coma, as diabetes or uraemia.