Answering your questions

Feb. 17, 2013

Editor's note: Anthony Kurec, MS, H(ASCP)DLM, answers questions from three readers. Anthony is Clinical Associate Professor, Emeritus, at SUNY Upstate Medical University in Syracuse, NY.

QWe recently had a physician request that anion gap be reported. What do most labs do—report every calculation they can (e.g., BUN/Creat ratio, A/G ratio, Indirect Bilirubin, Calculated Osmo, etc.)? And is there any type of “QC” associated with calculations (other than the annual review of the calculation formula)?

A Anion gap was initially used as a quality control parameter, but has become a common and useful monitor in managing acutely ill patients with metabolic acidosis.1,2 It is one of the first signs of metabolic acidosis and should be performed as part of electrolyte profiles.2  It provides a quick way to monitor treatment, whereby an increased anion gap (25-30 mEq/L) is seen with patients who are in diabetic acidosis or sepsis. A decreased anion gap (1-3 mEq/L) is consistent with elevated protein as seen in some malignancies, especially with a monoclonal paraprotein (multiple myeloma).3

An elevated BUN/creatinine ratio is useful in monitoring congestive heart failure, dehydration, and GI bleeding (especially when no vomiting of blood is present) and is decreased with liver diseases and malnutrition.1 These situations are common and may be a better indicator than the calculated GFR (glomerular
filtration rate).4

Some laboratories report albumin/globulin (A/G) ratios that can be helpful to a patient’s physician by alerting him or her to changes in protein levels.1 High ratios may indicate decreased immunoglobulin production as seen with certain genetic problems or in leukemias. Low ratios reflect liver or kidney problems.

Osmolality (# of moles of solute in a kg of water) can be measured by freezing point depression or by vapor pressure osmometers, but also can be estimated provided that some basic chemistries are available (i.e., Na, Gluc, BUN), and can be performed at the patient’s bedside (OSMO, CALC = 2 Na + Glucose/18 + BUN/2.8). For most patients this calculated value is not necessary when the laboratory has the appropriate instrumentation available. However, the presence of certain toxic materials in serum, such as methanol, ethylene glycol, isopropyl alcohol, or ethanol, can show a significant difference between the calculated osmolality and the measured osmolality, thus providing a useful screening tool for the presence of a toxic compound.6

Quality assurance related to use of calculated values is generally focused on the actual analyte(s) measured. If calculations are reported as part of the patient’s medical record, the quality check would be following the process of how the calculated value was obtained. This requires a log or other documenting record that can be provided to an inspector that clearly shows how that result was obtained for each patient.

In general, calculated ratios may serve as a screening tool and offer a quick alert to clinicians if there is a patient problem. However, follow-up of an abnormal ratio requires specific tests to be run. With today’s technology and instrumentation, extensive test menus offer quick and accurate measurements of analytes that are more specific to determining the exact abnormality.

The decision to include certain ratios may be of value, but physician education is required to ensure proper use and interpretation. This is a good topic to review at interdepartmental meetings to see what physicians want and/or need.


  1. Lab Tests Online. Accessed November 1, 2012.
  2. Klutts JS, Scott MG. Physiology and disorders of water, electrolyte, and acid-base metabolism. In: Burtis CA, Ashwood ER, Burns DE, eds. Tietz Fundamentals of Clinical Chemistry. 6th ed. 2008:655-674.
  3. Pincus MR, Abraham NZ. Interpreting laboratory results. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 22nd ed.  2011:91-108.
  4. Oh MS. Evaluation of renal function, water, electrolyte, and acid-base balance. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. 2011:169-192.
  5. Scott MG, LeGrys VA, Klutts JS. Electrolytes and blood gases. In: Burtis CA, Ashwood ER, Burns DE, eds. Tietz Fundamentals of Clinical Chemistry. 6th ed. 2008:431-449.
  6. Pincus MR, Abraham NZ. Toxicology and therapeutic drug monitoring. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. 2011:329-364.

QHas there been a recent article defining what fasting means?  Particularly, is black coffee or black/green tea allowed when drawing a fasting lipid panel?

AFasting prior to laboratory testing is generally considered to be 8 to 12 hours before the blood is drawn. It is almost always recommended that the patient NOT take anything other than water when told to fast before testing. This would include any teas or coffee, and especially if additives like milk or sugar products are included.

Coffee is a mixture of many different compounds including carbohydrates, lipids, nitrogenous compounds, vitamins, minerals, alkaloids, and phenolic compounds.1,2 Because of these various components, though they may make only a minor contribution to the overall cup of coffee, the amount taken may have some impact in laboratory results.

There is some evidence to suggest that coffee can affect HDL and LDL levels, as well as calcium and vitamin D levels in some individuals (depending on various factors: number and size of cups, filtered vs. unfiltered coffee, smoker, etc.).2 Polyphenols in coffee can bind nonheme iron, which can significantly affect its absorption and can inhibit bioavailable zinc absorption. Also associated with coffee consumption is an increase of the homocysteine level.2 Caffeine can elevate plasma free fatty acids, which can potentially alter certain laboratory test results.3

Green tea, like black and oolong teas, also contains caffeine, other alkaloids, and polyphenols, and thus may have effects similar to those seen with coffee.4  Green tea has been shown to lower total cholesterol and triglycerides and increase HDL.5 It may lower hemoglobin A1c in people with borderline diabetes. Vitamin K is also present and thus can have an effect on individuals on warfarin therapy.5,6 Green tea may also lower lithium levels.5  There have also been reported cases of individuals who have taken concentrated green tea extracts, resulting in liver problems.6

As a rule of thumb, it is best to abstain from any liquids other than water with no additives (sugar, lemon, flavored waters, commercial products such as Kool-Aid, MiO, etc.) for 8 to 12 hours prior to blood draw for best results.


  1. Al-Othman A, Al-Musharaf S, Al-Daghri N, et al. Tea and coffee consumption in relation to vitamin D and calcium levels in Saudi adolescents. Nutri J. 2012;11(1):56.
  2. Higdon JV, Frei B. Coffee and health: a review of recent human research. Crit Rev Food Sci Nutri. 2006;46:101-123.
  3. Sanford KW, McPherson RA. Preanalysis. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. 2011:24-36.
  4. WebMD. Find a vitamin or supplement: Green Tea. Accessed November 16, 2012.
  5. University of Maryland. Green Tea.  Accessed November 16, 2012.
  6. National Institutes of Health, National Center for Complementary and Alternative Medicine. Green Tea. Accessed November 16, 2012

QWe recently had a patient with macroglobulinemia. The patient’s serum viscosity was around 9 a few months ago, and is only down to 6.7 recently. My question is this: Is there any way the blood can be drawn and processed so that the viscosity will not cause sample probe clogs, and we might actually be able to give the physician some chemistry results? Or do we have no choice but to wait for the viscosity to drop more with treatment?

APatients with hyperviscosity syndrome have always been a challenge when it comes to laboratory testing. Consider use of another methodology/technique by reverting back to manual procedures. Qualitative strip testing for a limited number of analytes might be another approach.

There is a downside, however, in using non-routine methods that have not been properly validated. Most laboratories have long since given up many of the manual methods of the past; thus re-implementing reference intervals, quality control expectations, and other regulatory requirements would be a major undertaking dedicated to a very limited number of patients.

Another approach that would be relatively easy to try is diluting the patient sample. Depending on the analyte, a highly viscous serum sample can be diluted with the same diluent used from the instrument. A dilution of two- or three-fold may be adequate to process the specimen properly without clogging sample probes.

MLO’s “Tips from the clinical experts” column provides practical, up-to-date solutions to readers’ technical and clinical issues from experts in various fields. Readers are encouraged to send questions to [email protected]