Another interesting point elicited by Pratt was that in his _gouty_ examples, although, after a purin meal, the uric acid content of the _blood_ rose markedly, yet there was no apparent _delay_ or _diminution_ in the output of _exogenous purin_ in the _urine_. Thus, in one example, after the intake of 190 grams of thymus gland, the uric acid in the blood, in the first twenty-four hours, rose from 2·2 mg. to 4·4 mg., reaching, on the third day, a maximum of 8·7 mg., which, on the fourth day, sunk to 2·7 mg. Nevertheless, 26·2 per cent. of the ingested purin nitrogen was excreted as uric acid. Now, as pointed out in the preceding chapter, it has been shown by many observers that in _gouty_ subjects the excretion of _exogenous_ purin is _diminished_ and _retarded_. But Pratt’s study of the blood shows that a marked increase and retention of uric acid in the _blood_ may result from the ingestion of purin bases, even when no evidence of retention is found on the examination of the _urine_.[23] The clear inference from this is that it is desirable that our _urinary_ findings in respect of _uric acid_ should be reviewed and controlled in light of _blood examinations_ to the same end. VARIATIONS IN URIC ACID CONTENT OF BLOOD INDEPENDENTLY OF DIET Considerable variations in the uric acid content of the blood, according to Pratt, may occur both in _gouty_ and _non-gouty_ subjects, and which cannot be attributed to any _purin intake_. Such oscillations, moreover, may ensue within a short time. A patient of his, admitted to hospital suffering from a severe attack of _gout_, was placed upon a _purin-free_ diet. Twenty-four hours afterwards examination revealed only 2·7 mg. of uric acid in his blood. Subsequently, after having had no food containing purins for fifteen days, it contained 5·1 mg. Marked variations in the uric acid content of the blood may likewise occur in _non-gouty_ subjects. After being on a _purin-free_ diet for two days, a patient of Pratt’s, with _recurrent iritis_, had 2·2 mg., while a few months after, when on a mixed diet, his blood contained only 0·8 mg. Again, great oscillations in the blood content of uric acid, independent of diet, are sometimes found in cases of _non-gouty arthritis_. Thus, in one chronic case of this nature, the blood when first examined contained 7·6 mg. of uric acid, but a few months later, when on a purin-rich diet, only 0·8 mg. were present. In another instance of _primary polyarthritis_ the same was strikingly exhibited. Aged twenty-two years, the subject in October was on ordinary diet. His blood at that period showed 2·7 mg. of uric acid per 100 mg. of blood; in December, on a purin-free diet, 5·0; and in May, on a similar dietary, 1·6 mg.