Measurement

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Questions Received:

  1. Chemical pathology reports contain values expressed in mmol/l. What exactly is a millimole?

  2. Can you please explain the difference between osmolarity and osmolality?

  3. When taking temperature under the arm, is the true reading one degree higher or lower than what the thermometer reads?

  4. What is the normal haemoglobin count in 100cm3 of blood?

  5. What are cardiolipins i.e. IgM cardiolipin and IgG cardiolipin? What is the pathological significance of results higher than normal U/ml 0-7.2?

  6. Please define the term "total osmoles" as it relates to measurement of "total osmoles" from a 24 hour urine collection. What are the units of measurement and how is it calculated?

  7. When a blood test shows a higher than acceptable level of alkalne phospherous what conditions are possible from this result? I’d appreciate your input.

  8. What is the molecular weight of cholesterol. I am trying to convert the American expression of mg/dL for cholesterol readings into the British expression of mmol/L.

Responses:

Chemical pathology reports contain values expressed in mmol/l. What exactly is a millimole?

30th November 1998

A mole is a standard unit used when describing the amount of a substance, and is the molecular weight of the substance expressed in grams. One millimole is a thousandth of a mole, so a value expressed in mmol/l is giving information about the concentration of a solution.

To understand the rationale behind this measurement, we have to remember that all material things - including solids, liquids, and gases - are made of tiny atoms and molecules. Although they are tiny, it has been possible to measure the atomic weight of each element accurately. Some atoms are comparatively light, for example hydrogen, and others are comparatively heavy, for example iron. The same is true of molecules made up of combinations of atoms - some molecules are heavier than others. So when it comes to making up solutions, account has to be taken of these differences in weight of the individual atoms and molecules.

First of all, the atomic or molecular weight of the substance is determined. So for sodium chloride (common salt) the molecular weight is 23 (the atomic weight of sodium) plus 35.5 (for chlorine) which makes a total of 58.5. Then, the molecular weight is expressed in grams: 58.5 grams in the case of sodium chloride. This is known as a mole of the substance, and will be different for different substances.

A mole of one substance has the same number of atoms or molecules as a mole of any other substance. This allows us to standardize and compare the properties of solutions, which is important when dealing with physiological processes. The molarity of a solution is the number of moles of a substance dissolved in one litre of solution. Thus, a one molar solution of sodium chloride (written as 1M NaCl) has one mole (58.5 grams) of sodium chloride in one litre of solution.

In the body, the concentrations of substances in the various body fluids are generally much less than 1M, and are more conveniently expressed in the smaller millimolar units per liter of solution. It is the number of these that is measured and then stated on a chemical pathology report - mmol/l. Expressing values in this way became internationally recognised in the mid-1970s when the System Internationale was introduced and has replaced measurement in other units such as milliequivalents per litre (mEq/l).

Can you please explain the difference between osmolarity and osmolality?

2nd December 1998

There is no difference! Osmolarity tends to be the preferred term here in the UK, but both words are in common use in the literature. They are both derived by multiplying together the molarity of a solution and the number of particles into which each molecule of the solute dissociates in solution. So, in the case of glucose which does not dissociate in solution, a one molar solution (180 grams of glucose dissolved in one litre of solution) has an osmolarity of one osmole. On the other hand, when sodium chloride molecules dissolve in solution they dissociate into two ions - a sodium ion and a chloride ion - and so a one molar solution (58.5 grams dissolved in one litre of solution) has an osmolarity of two osmoles. The significance of this stems from the way that osmotic pressure is generated by the number of solute particles in a solution, not so much their size or chemistry. Thus, the greater the number of particles into which a substance dissociates, the greater the osmotic pressure it will generate for a given concentration (molarity).

OsmosisDiagram to illustrate osmosis - water passes through the semipermeable membrane into the solution on the other side. The presence of particles of solute in the solution creates this osmotic pressure.

 

 

 

 

 

When taking temperature under the arm, is the true reading one degree higher or lower than what the thermometer reads?

12th April 1999

The reading obtained from the axilla is approximately 1 degree Celcius below the core temperature of the body. Measurements made in this location tend to be less accurate than those made in from the mouth or rectum, but can be valuable in some circumstances. Being non-invasive it is ideal when making measurements in newborn babies, for example.

What is the normal haemoglobin count in 100cm3 of blood?

13th April 1999

There are slight differences in the figures given by different sources, but the normal ranges are generally taken as follows:

What are cardiolipins i.e. IgM cardiolipin and IgG cardiolipin? What is the pathological significance of results higher than normal U/ml 0-7.2?

29th April 1999

Cardiolipin belongs to the family of molecules known as phospholipids which help to form important cellular components such as membranes. IgM and IgG are different categories of antibodies that are made by the B cells of the immune system, usually in response to antigens which are not within the normal range of antigens carried by cells in the body (‘non-self’ antigens compared with ‘self’ antigens). So for example, when there is a bacterial or viral infection, or when normal cells of the body become transformed into cancer cells, the immune system responds by making specific antibodies against the non-self antigens. Sometimes, however, the immune system begins to make antibodies against normal constituents of the body. This is called an autoimmune response, and can give rise to a number of diseases. Anti-cardiolipin antibodies have been shown to be associated with blood-clot formation (thrombosis) in a range of autoimmune conditions. For example, systemic lupus erythematosus patients have a disposition to spontaneous abortion, thrombocytopenia and thrombosis. The term "anti-phospholipid syndrome" has been introduced to describe patients who present with these clinical manifestations in association with anti-cardiolipin antibodies in the IgG and IgM categories.

There are several test kits available for the assay of anti-cardiolipin antibodies, and slightly different ranges of normal values are quoted for each of these tests. Calibrator sera are provided, with the IgG and IgM anti-cardiolipin concentrations expressed in GPL or MPL units respectively. These units are traceable to the internationally recognized preparations of the Phospholipid Standardization Laboratory, University of Louisville. The values are an aid to diagnosis, and should be interpreted in the light of the patient's history, physical findings, and other diagnostic procedures. If the values are higher than normal, then it is evidence of an autoimmune response and this may help in the interpretation of the clinical signs and symptoms.

Relevant Publications

Please define the term "total osmoles" as it relates to measurement of "total osmoles" from a 24 hour urine collection. What are the units of measurement and how is it calculated?

26th April 1999

"Total osmoles" is a measure of the total amount of dissolved substances (solutes) in a given volume of fluid. In the case of urine, there are many solutes present, including sodium ions, potassium ions, chlorine ions, urea, creatinine, and uric acid. If urine is collected over a 24-hour period, as for example when information is required about kidney function, a particular volume is obtained, usually somewhere in the range of 800 to 1500 ml. Therefore, to calculate the total osmoles in the sample it will be necessary to measure its concentration and then multiply by the volume obtained.

The concentration of the urine can be estimated by measuring its specific gravity using a simple flotation device. However, this is not a particularly accurate measure and a more precise result can be obtained by measuring freezing point depression, or vapour pressure lowering, or by measuring the pressure required to prevent water passing through a semipermeable membrane into the sample of urine.

It is worth recalling that the concentration of a solution can be expressed in several ways, for example as a given weight of solute in a given volume, or in moles (where a mole is the molecular weight of the solute expressed in grams), or in osmoles if the substance dissociates in solution. The osmolarity of a solution is the molarity multiplied by the number of parts into which each molecule of the solute dissociates.

The concentration of the urine will be affected by several factors, for example: the level of hydration of the person, the amount of circulating anti-diuretic hormone (ADH), and the efficiency of the kidneys. Urine is usually slightly more concentrated (hyperosmotic) than plasma and tissue fluid, which are generally about 300 mOsm/L (where mOsm means milliosmoles, or thousandths of an osmole). Under the influence of ADH, and with normal kidney function, urine concentration can rise to as much as 1200 to 1400 mOsm/L.

So, to calculate the total osmoles in a 24-hour sample of urine, measure the volume of the sample in litres, measure its osmolarity, and then multiply the two together to obtain the total number of osmoles present. The units will generally be milliosmoles.

When a blood test shows a higher than acceptable level of alkalne phospherous what conditions are possible from this result? I’d appreciate your input.

18th June 1999

A raised level of alkaline phosphatase in the blood indicates that something may be wrong, although sometimes the rise is transient and the level returns to normal after one to three months without a clinical cause being found (Lieberman and Phillips, 1990). The the wide range of clinical conditions associated with raised alkaline phosphatase include liver disease, heart problems, benign bone disease, treatable malignancy, sepsis, and acquired immunodeficiency syndrome - AIDS (Price and Sammons, 1976; Payne et al, 1991; Maldonado et al, 1998).

If the alkaline phosphatase level remains high then usually it is associated with clear clinical signs. Thus a reasonable approach in this situation is for the doctor to take a careful clinical history, make a physical examination, and carry out routine laboratory studies to reach a diagnosis, followed by another measurement of the alkaline phosphatase level after one to three months (Lieberman and Phillips, 1990).

References

What is the molecular weight of cholesterol. I am trying to convert the American expression of mg/dL for cholesterol readings into the British expression of mmol/L.

2nd August 2001

The molecular weight of cholesterol is 387. Normal values for cholesterol depending upon age are (Lewis et al, 1992):

Conventional Units SI Units
140-220mg/dL 3.6-5.7mmol/L

Conversion: multiplying the mg/dl by 0.0259 enables the mmol/L to be obtained.

Reference

We should like to thank Mr. Stephen Bottomley, Biochemist, for his help with this response.

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