Abstract
Background and Aims
To present the history of the specific gravity test in the study of urines from the 15th cent. AD until today.
Method
The first description of a hydrometer, the progenitor of the urinometer, appears in the fifteenth letter from Synesius of Cyrene to the Greek scholar Hypatia of Alexandria, from the 5th century AD. However, its evolution to the urinometer took 15 centuries until 1849, when Johann Florian Heller (1813-1871) introduced it. During the next two centuries, we note a rapid improvement of the appropriate laboratory instruments and a wide acceptance of the importance of specific gravity for the evaluation of urine. We present passages from relevant texts from the discussed period.
Results
The first call for attention to measuring urine weight comes from Nicolas of Cusa’s work The Layman: Experiments with Weights, where we read “Accordingly, since the weight of blood or the weight of urine is different for a healthy man and for a sick man or for a youthful man and an elderly man or for a German and an African, wouldn’t it be especially useful to a physician to have all these differences recorded? Orator: Most certainly”. The theoretical proposal was solidified in Francis Bacon’s Novurn Organum (1620): “Neither the naked hand nor the understanding left to itself can effect much. It is by instruments and helps that the work is done”. Herman Boerhaave, in 1753, weighed the distilled urine residue in order to calculate its density, a complicated and time-consuming procedure. Later, Groenevelt J. in The Rudiments of Physick writes “The particles in it sink or float according to their own gravity [specific!], but their position is also dependent on the thickness of the urine [albumin in it plus formed particles].” Osborne, in 1820, made a genius comparison “When a mucous cloud is present [in the urine] it ascends and descends in the fluid according to specific gravity, thus serving the purpose of a hydrometer”. Becquerel, in 1842, laid out tables calculating the solid articles in urines of given specific gravity. Graves, in 1866, reported intermittent albuminuria with a parallel variation of the specific gravity of urine in infectious disease. The same year, an article in the London Medical Gazette: Or, Journal of Practical Medicine, and another one in The New York Lancet stress the variation of specific gravity in the course of a renal disease. Clover R.M. in the Case of sialorrhoea (1846) states that specific gravity may vary greatly even with the same amount of solids and fluid due to fluid heterogeneity. In 1848, Garrod describes a bottle for measuring urine specific gravity. Johann Florian Heller introduced the mercury-based floating urinometer in 1849 thus greatly facilitating the test of urine specific gravity.
The increasing demand for the test’s application resulted in the discovery of refined laboratory instruments, such as the refractometer. Very recently, urine specific gravity is considered an accurate renal function marker, equal to creatinine clearance or proteinuria levels (Constantiner M. et al, Am J Kidney Dis., 2005 May;45(5):833-41. and Anestis SF et al, Am J Primatol., 2009 Feb;71(2):130-5).
Conclusion
It seems that the understanding of urine specific gravity underwent “mechanisation”, from its inception as the use of the sediment’s location in the vial to access it, during the Middle Ages to complex apparatuses like refractometers. Originating in the Classical and Middle Ages mainly from the East, it obtained its sound scientific background in the West from the Renaissance onwards. After debate on its usefulness, it is again a vital tool for assessing renal damage.
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