Kaplan: Clinical Chemistry, 5th Edition
Clinical References - Methods of Analysis
nxxOsmolalitypathetic
Goce Dimeski i
Name: Osmolality
Clinical significance: click here
Refer to Chapter 30, Renal Function, in the 5th edition of Clinical Chemistry: Theory, Analysis, Correlation.
Students’ Quick Hyperlink Review
•Principle of analysis and current usage
•Formulae for calculating osmolality
•Reference and preferred methods
•Specimen
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•Interferences
•Reference intervals
•Interpretation
•Performance goals
•References
•Procedure: Osmolality by freezing-pint depression
Principles of Analysis and Current Usage
Osmolality is a colligative property of solutions that depends on the number of dissolved particles prent in the solution. The three types of solutes most often encountered in biological fluids are electrolytes, organic molecules, and colloids. In healthy persons, the principle contributors to the measured osmolality are Na+, Cl−, and bicarbonate. In persons with diabetes or renal failure, glucos
冬季安全小常识e and urea may also contribute significantly to the measured osmolality. As the number of dissolved particles increas, the freezing point and vapor pressure of a solution are decread, and the osmotic pressure and boiling point are incread [1].In dilute solutions, there is a linear change in the colligative properties as the solute concentration increas. It is important to realize that it is not the mass concentration but the molal concentration (that is, moles per kilogram of solvent) that is the basis of colligative properties.
i Osmolality
Previous and current authors of this method:
First edition: Lawrence A. Kaplan
Methods edition: Lawrence A. Kaplan
Second edition: Lawrence A. Kaplan
硫化氢的性质Third edition: Steven C. Kazmierczak, Lawrence A. Kaplan
Fourth edition: Lawrence A. Kaplan
Fifth edition: Goce Dimeski
For example, in biological fluids the concentration of salts and low-molecular-mass organic compounds (such as gluco and urea) affect osmolality far more than albumin, which is prent in a large mass amount but becau of its large molecular mass is prent at low molal concentrations. The molar concentration of plasma albumin, when prent at 40 g/L, is approximately 0.58 mmol/L. In contrast, the molar concentration of plasma NaCl, when prent at 5.8 g/L, is approximately 150 mmol/L. Normal plasma is approximately 93% water, with the remaining 7% being solids (proteins, lipids, etc). The electrolytes are dissolved in the water component. Since there are 2 osmotically active components in NaCl (Na+ and Cl−), the osmotic contribution is 0.93 × 2, or 1.86 times the concentration of Na+ (2 or 1.86 accounting for Na+ and Cl−, with Cl−being the major accompanying anion).
The most frequently measured colligative property ud to estimate fluid osmolality is freezing-point depression. When 1 mole of solute is added to 1 kg of water, the freezing point is depresd by 1.858°C. The laboratory osmometer actually employs a nsitive heat thermistor to measure the heat relead by the freezing fluid and relates it to the freezing point of the fluid.
An infrequently measured parameter is vapor-pressure depression. The vapor-pressure osmometer actually measures the dew-point depression of the vapor, that is, the vapor in equilibrium with the solution being measured. The more dissolved particles prent (incread osmolality), the lower the vapor pressure of the aqueous component of the solution. An important exception to this is found when the solute itlf is a volatile substance.
Formulae for Calculating Osmolality长春招生
A “stat” procedure frequently ud to estimate the osmolality of a fluid is the calculated osmolality. To calculate rum osmolality, one sums up the molar concentrations of tho substances which contribute most to the measured osmolality—namely, sodium, gluco, and urea. The contribution of chloride and bicarbonate is handled by multiplying the sodium concentration by a variable factor as discusd below [2].
Although there are many equations ud to calculate osmolality, the simplest and most frequently ud is:
Eq. 1[3]
Calculated mOsm/L H
O = 2 [Na+] + gluco (mmol/L) + urea (mmol/L)
we will rock you歌词2
or
pleasure是什么意思Calculated mOsm/L H
O = 2 [Na+] + gluco (mg/dL) + BUN (mg/dL)
2
2.8
18
where sodium concentration is expresd in milliequivalents per liter (mEq/L) or mmol/L, and the concentrations of gluco and urea or blood urea nitrogen (BUN) are divided by their respective formula equivalent weights, 180 and 28, to convert the mass concentrations into millimoles per liter (mmol/L) [4]. This formula is propod to balance simplicity and ea of calculation with the goal
of obtaining results that cloly compare to a measured osmolality. Other more accurate formulas that have been suggested include the following two:
Eq. 2[5]
Calculated mOsm/L H
O = 1.86 [Na+] + gluco (mmol/L) + urea (mmol/L) + 9
2
or
O = 1.86 [Na+] + gluco (mg/dL) + BUN (mg/dL) + 9
Calculated mOsm/L H
2
18 2.8
Eq. 3[6]
O =
Calculated mOsm/L H
2
1.86 [Na+] + 1.38 [K+] + gluco (mmol/L) + urea (mmol/L) + 7.45
or
O =
Calculated mOsm/L H
2
1.89 [Na+] + 1.38 [K+] + 1.08[gluco (mg/dL)] + 1.03 [BUN (mg/dL)] + 7.45
18 2.8
Rasouli and Kalantari [7] have suggested formula 3 for computer calculations on automated analyzer
s. There are many other formulas suggested for calculation of osmolality [2,6,8,9,10,11]. It is important to note that although osmolality should be expresd as milliosmoles per kilogram (mOsm/kg) of water, it is most frequently calculated and reported as milliosmoles per liter (mOsm/L) of water. In dilute solution like plasma at normal temperature and pressure, the osmolality and osmolarity are effectively the same. The slight error introduced due to the difference between the two expressions has little clinical significance [5]. The 9 or 7.45 mOsm/kg is added to reprent the contribution of other osmotically active substances in plasma such as K+, Ca2+, Mg2+, and their unmeasured anions and proteins.
The calculation of urine osmolality is less accurate than plasma [12]. The commonest formula is Urine osmolality = 2 × (Na+ + K+) + Urea (mmol/L). Other formulae may include in the calculation the gluco that may be prent in some urine samples, namely from diabetic patients. The prence of gluco, ammonium, or foreign substances such as mannitol will result in underestimation of the calculated urine osmolality [13].
Fecal osmolality is infrequently requested in investigations of chronic idiopathic diarrhea. Calculated fecal osmolality should be ud in preference to measured osmolality, which increas after defecation due to continued bacterial fermentation [14]. The Na+ and K+ are measured in the fecal
water, and their sum is doubled to account for anions: Calculated fecal osmolality = 2 (Na+ + K+).
韩国的英文怎么写A 2007 College of American Pathologists (CAP) quality assurance survey report indicated that the vast majority (97.9%) of laboratories ud a freezing-point depression osmometer. It should be pointed out that nearly all automated analyzers now determine a calculated rum osmolality, or it can be calculated by laboratory information systems. Thus the most frequently reported rum osmolality is a calculated rum osmolality.
Reference and Preferred Methods
Although there is no reference method for osmometry, the freezing-point depression method is considered the de-facto reference method.
It has been pointed out that the two types, vapor-pressure and freezing-point osmometers, differ in their ability to respond to volatile solutes that may be clinically encountered [15,16].The contribution to rum osmolality of veral commonly en alcohols (e.g., ethanol, methanol, and isopropanol) is directly proportional to their molar concentrations when osmolality is measured by freezing-point depression osmometry.However, vapor-pressure osmometers do not detect the volatile substances, becau volatile solutes contribute to the total vapor pressure prent above the solutio
n, though they decrea the vapor pressure of the water portion of the solution [17]. Therefore, the decread vapor pressure of water is counterbalanced by the increa in vapor pressure caud by the prence of a volatile substance. As the concentration of the volatile substance increas in aqueous solutions, the vapor pressure above the solution actually increas, giving fally low osmolality results.
Apart from the problem encountered with volatile solutes, the osmolality of most specimens measured by freezing-point depression correlates well with the osmolality measured by vapor-pressure depression. In practice, vapor-pressure osmometers are ldom ud. Becau of its greater sturdiness and precision, most laboratories u the freezing-point osmometer.
The calculated osmolality is not meant to provide a definitive result but only one sufficient for most acute emergency rvice work. Ud as such, the calculated osmolality can provide clinicians with valuable information with no extra analysis time. The error introduced by calculation of the rum osmolality as mOsm/L instead of mOsm/kg can be clinically significant only if the percentage of water in the sample (approximately 93%) changes significantly. Thus highly lipemic samples or samples with large amounts of protein (as from multiple-myeloma patients) can have an inaccurate calculated osmolality.The laboratory might not wish to report a calculated osmolality on such specim
ens or alternatively might affix a comment to the results warning the physician of a possible error. The calculated osmolality will also give fally low results in the prence of volatile solutes. Additionally, the inclusion of K+, Ca2+, or Mg2+ concentrations in formulas has not resulted in a more accurate calculated osmolality. Specimen
Blood drawn without the u of any anticoagulant is the preferred sample. The rum should be removed from the clotted blood cells as rapidly as possible. Well-centrifuged urine or other body fluids are also acceptable samples. Random urine samples can vary considerably in the concentration of analytes and are rarely uful clinically. Therefore an aliquot of a 24-h urine collection is the preferred specimen for the measurement of urine osmolality.
Any fluid left uncovered will have an incread osmolality becau of evaporation and concentration of sample solutes. Serum osmolality has been reported [18] to be stable for 3 days at room temperature, 14 days at −7°C, 14 days at −21°C, and as long as 56 days at −78°C. Urine osmolality may be stable at 4°C for up to 24 h [19]. Urine samples should be centrifuged to remove particulate matter.
Fecal specimens requested for osmolality estimation are uful in diagnosis of watery diarrhea. Sam
ples require centrifugation, and the supernatant should be ud for the Na+ and K+ measurement. Dilution of solid fecal samples in water is not recommended [14]. Interferences
As previously mentioned above, volatile substances such as alcohols cau a significant bias in osmolality measured using vapor-pressure depression osmometers. U of citrate as an anticoagulant has been found to significantly increa measured osmolality [20]. Hemolysis, even when prent in gross amounts, has been found not to interfere with osmolality measured by u of freezing-point depression [21].
Any particulate matter, microclots in rum/plasma, particulate matter in urine, feces, or other fluid types are capable of altering the freezing or vaporizing process, thus leading to inaccuracies. Therefore, all samples should be free of such particulate matter and may require centrifugation prior to analysis.
Osmolality Reference Interval
diaoyu island
The reference interval for rum osmolality in a healthy person, as measured using a freezing-point depression osmometer, is 280 to 295 mOsm/kg [22]. The approximate contribution from osmotically active substances is as follows: Na+ 140; Cl− 105; HCO3− 27; K+ 4; Gluco 5; Urea 5; Ca2+ 2; Mg
2+ 1; HPO42− 1; protein 1 and organic compounds ~5. Individuals living at high altitude (i.e., > 3800 m) have been found to have significantly lower rum osmolality compared to individuals living at a level [23].
Urine osmolality can range from 50 to 1000 mOsm/kg [13,24].
Interpretation
Osmolality measurements are most frequently ud to help determine whether a patient is in a hyperosmolal state. Such states can be en in renal failure (hyperuremia), diabetes (hyperglycemia), and dia states in which there is water loss in excess of solute loss. In the latter clinical states, the degree of hyperosmolality parallels the degree of hypernatremia. Hyperosmolality due to urea is far less dangerous than when due to gluco. Urea freely diffus across cell membranes, resulting in concentration equilibrium. Gluco does not diffu across cell membranes and caus water movement out of cells. Hyperosmolality due to hyperglycemia is often associated with acute illness, shock, and renal failure, including deficit in body potassium.
The u of the osmolal gap (the difference between calculated and measured osmolality) to screen for the prence of exogenous substances can be very uful in an emergency room situation. It mu
st be pointed out that the osmolal gap may depend less on the equation employed than on the population tested. For example, in patients with incread unmeasured solutes, such as patients in circulatory shock, renal failure, in very low birth weight neonates, hyperosmolal nonketotic coma, alcoholic ketoacidosis, diabetic acidosis (due to increa in ketone
