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  Q & A

 

 

 

 

March 2012

Editor:
Fredrick L. Kiechle, MD, PhD

Question Q. Our laboratory provides services to a nursing home, and we have run into problems with early-morning blood draws for determination of hematocrit in a group of patients. In some patients if the blood is drawn early in the morning before the patient is out of bed, the hematocrit determination can be aberrantly low. This sometimes results in the patient being transported to the hospital for evaluation, with repeat hematocrit showing a normal result. We have attributed these findings to postural pseudoanemia (Mayo Clin Proc. 2005;80:611–614). The nursing home medical staff is convinced the results are due to laboratory error related to transportation time or technical problems. We have reviewed these cases and found no technical problems. The samples with the low hematocrit result have been repeated also in the hospital laboratory with the same results. Is hemodilution a legitimate explanation?

A. It is inappropriate to take the dismissive attitude that differences in sequential laboratory results are caused by laboratory error. Though laboratorians are well aware of the improbability of laboratory error, many clinical caregivers, because of a lack of appropriate training, believe that results they do not like must be laboratory error. Evaluating preanalytical variables, including blood draw technique and other factors that can affect laboratory results, is an important part of interpreting feasible physiological variances within patients.

Although hemodilution may be a legitimate explanation for sample variability, it is not the only explanation. In a study of seven healthy men, Hagan, et al.,1 showed that the red cell volume did not change significantly with a change from a supine to a standing position, whereas the blood and plasma volumes decreased by about 600 mL on average. Typically, it takes about 30 minutes in either position for the measurements to stabilize, though Hagan, et al., demonstrated that maximal hemodilution and hemoconcentration occurs at 40 and 60 minutes, respectively, in supine and standing positions.

In the study conducted by Jacob, et al.,2 changes in plasma volume in different individuals with movement from lying to standing varied from six percent to 25 percent. The changes from lying to sitting are less than when an individual moves from lying to standing.3 Postural changes are greater in hypertensive individuals, because of increased hydrostatic pressure, and in those with cardiac insufficiency. Changes also tend to be greater in those who are less than optimally nourished because of reduced plasma oncotic pressure. Such conditions may be observed in the typical elderly nursing home resident, regardless of any medical condition. The magnitude of the variations reinforces the need to standardize specimen collection conditions.

Additional factors to consider when looking at the differences between successive hemoglobin and hematocrit results are the possible presence of hemolysis in the specimen, tourniquet applications of different durations (tourniquet use induces hemoconcentration), and analytical variability.

References

1. Hagan RD, Diaz FJ, Horvath SM. Plasma volume changes with movement to supine and standing positions. J Appl Physiol. 1978;45:414–417.

2. Jacob G, Raj SR, Ketch T, et al. Postural pseudoanemia: posture-dependent change in hematocrit. Mayo Clin Proc. 2005;80:611–614.

3. Maw GJ, MacKenzie IL, Taylor NA. Redistribution of body fluids during postural manipulations. Acta Physiol Scand. 1995;155:157–163.

Donald S. Young, MD, PhD
Department of Pathology and Laboratory Medicine
Hospital of the University of Pennsylvania
Philadelphia

Question Q. It has been reported that while serum creatinine is specific for renal disease, its sensitivity is rather poor; once the creatinine is elevated, two-thirds of renal function has been lost. Also, creatinine levels vary directly with the patient’s muscle mass. Serum cystatin C, an endogenous substance that is cleared by glomerular filtration, is reported to be much more sensitive than creatinine. If the serum cystatin C is elevated, is there a scale that can be used to correlate that value with an estimate of residual renal function (for example, glomerular filtration rate)?

A. Creatinine and cystatin C have been reported to have equivalent usefulness in detecting deteriorating kidney function and predicting risk for progression of chronic kidney disease because both biomarkers have an incremental change in an individual that reflects changes in GFR function.1,2 The perception that creatinine is less sensitive relates to the use of a conventional reference interval to interpret results. The typical upper limit for a reference interval, depending on the age of a patient, corresponds approximately with a GFR of 60 mL/min/ 1.73 m2, which is loss of about half of a person’s kidney function. If, on the other hand, GFR is estimated from creatinine, the relationship to kidney function will be represented more appropriately because the estimating equations partially correct for age, gender, and race factors. The Modification of Diet in Renal Disease (MDRD) equation for estimating GFR should not be used to report a numeric value greater than 60 mL/min/1.73 m2 because this equation underestimates GFR at higher values. The newer Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation is more accurate at larger eGFR values and can be used to estimate GFR at values closer to normal and, thus, to track progression of chronic kidney disease.3,4

An important limitation of using creatinine as a biomarker for kidney function is creatinine’s dependence on muscle mass. Any condition that affects muscle mass (including frailty in the elderly, critical illness, or cancer) will decrease the usefulness of creatinine in assessing kidney function. In addition, creatinine is less useful for patients with unusual diets and those with conditions associated with altered secretion or extra-renal elimination of creatinine. The limitations of using creatinine are described more fully on the National Kidney Disease Education Program Web site.5

Cystatin C is a promising biomarker for kidney disease that can be used in conjunction with creatinine and, in particular, under conditions when creatinine is less well correlated with kidney function. Cystatin C is produced by nucleated cells throughout the body and is unaffected by muscle mass and age above one year. The limitation of using cystatin C is that the measurement procedures are not standardized and produce different values for the same patient sample. Consequently, equations that have been developed to estimate GFR from cystatin C are suitable only when used with the measurement procedure and for a population similar to that used to develop the equation.

A workgroup of the International Federation of Clinical Chemistry and Laboratory Medicine has developed a reference material for cystatin C that is available from the Institute for Reference Materials and Measurements in Belgium. This serum-based reference material is in the process of being characterized for commutability with patient samples among commercial measurement procedures to enable manufacturers to standardize calibration traceability to this reference material. The workgroup is also developing a universal equation to estimate GFR from standardized cystatin C results based on a large and diverse population. When these activities have been completed, cystatin C will be a useful addition to the tests available to monitor kidney disease and be particularly useful for patients in whom the relationship between creatinine and kidney function is compromised.

References

1. Spanaus KS, Kollerits B, Ritz E, et al. Serum creatinine, cystatin C, and beta-trace protein in diagnostic staging and predicting progression of primary nondiabetic chronic kidney disease. Clin Chem. 2010;56:740–749.

2. Dalton RN. Serum creatinine and glomerular filtration rate: perception and reality. Clin Chem. 2010;56:687–689.

3. Levey AS, Stevens LA, Schmid CH, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med. 2009;150:604–612.

4. www.nkdep.nih.gov/labprofessionals/CKD-EPI_equation.htm.

5. www.nkdep.nih.gov/labprofessionals/.

Greg Miller, PhD
Professor of Pathology
Director of Clinical Chemistry
Director of Pathology
Information Systems
Virginia Commonwealth University
Richmond

Question Q. I practice in a small community hospital as part of a two-pathologist group. In our practice we rarely see liver core biopsies. When we do, it is usually in the setting of a mass lesion. Occasionally, we are asked to evaluate the suitability of a potential donor liver for transplantation. In the interest of the health care of the recipient, involving the pathologist at the tertiary hospital in this decision is the more appropriate practice, in my view. For medicolegal reasons, I refuse to perform any frozen sections of the liver for this usage. I would appreciate your opinion.

A. I feel that this approach is appropriate, particularly if handling donor frozen sections is something you do not do regularly or your comfort level is low, or both. If you are a pathologist who does not typically handle these types of cases, being involved in deciding whether to use a potential donor can be difficult.

Several points can justify this position. At most transplant centers, the decision whether to use a potential allograft is based on multiple pieces of information, including, among other things, history, serologic studies, and recipient status. These data may not be available at the time of organ procurement and can make it difficult to determine the significance of any questionable changes seen at frozen section. In addition, most centers will overread outside slides or obtain their own biopsy before proceeding with the transplant. Finally, the decision whether to use a graft is often made at the receiving hospital, especially in cases in which there is question about the donor’s organ status.

Geoffrey A. Talmon, MD
Assistant Professor
Department of Pathology and Microbiology
University of Nebraska
Medical Center
Omaha


Dr. Kiechle is medical director of clinical pathology, Memorial Healthcare, Hollywood, Fla.