Again, though we speak of uricæmia as a dominant characteristic of gout, we are uncertain whether the alleged increase in the uric acid content of the blood is _real_ or merely _apparent_. We can, it is true, extract _uric acid_ and _urates_ from the blood-stream, but it does not necessarily follow that it is as such that they circulate _in vivo_. We need walk circumspectly here for, despite the most modern methods of blood analysis, we are still ignorant as to the exact form in which uric acid exists in the blood-stream; whether the purins of the food appear in the blood-stream as _sodium monourate_, or in _organic fusion_. Accordingly, in the interests of progress, it were well to bear in mind the pitfalls that beset uric acid estimation, the insufficiently eclectic capacity of even the most modern tests, and to consideration of these more chastening aspects we now proceed. _Sources of Fallacy in Uric Acid Estimation._—With Folin’s findings as his basis, Walker Hall estimates that, excluding the lymphatics and lymph spaces, the entire blood-stream contains normally 70 mg. of uric acid, _i.e._, 2 mg. of uric acid per 100 grams of blood, 3,500 c.c. (total quantity of blood). Thence he argues that, inasmuch as about 1 litre of blood traverses the kidney per minute, the total content thereof of _uric acid_ would gain access to the renal organs in three and a half minutes. Now the average total output of the kidneys is 500 mg. per twenty-four hours. Accordingly, assuming that the blood arriving at the kidneys contains as a constant the above 70 mg. uric acid, the total daily output would pass through these organs in twenty-five minutes. Now, given immediate extraction of all the uric acid by the renal cells, then the blood in the renal veins will become _free of uric acid_. If so, the estimates of the uric acid content of the blood will reflect exactly the measure of the _endogenous_ or _exogenous_ nuclein metabolism. But, “if the renal vein blood is not _purin-free_, then the _estimations will fail to yield a true picture of the activities of nuclein exchange_.” Again, as to the precise import of isolated estimates of the uric acid blood content, we must recollect that the _excretion of purins_ is not distributed evenly over the twenty-four hours, varying as it does under the influence of food, exercise, sleep, and other factors. _A propos_ of this, Pratt’s observations clearly show that both in _gouty_ and _non-gouty subjects fluctuations in the uric acid blood content_ also occur, and this independently of _diet_. To what, then, may these variations be referred? Obviously a question of great moment, especially when we recall the eccentric behaviour of the _blood uric acid_ in relation to the incidence of _acute gouty attacks_. For, until the inward meaning of these vagaries is revealed, the value of recorded estimates must necessarily be discounted considerably.