4.23: Phlebotomy

PROBLEM

During a third shift, the laboratory receives a request for a stat. renal profile on a serum sample. The patient is a 35 year old white male. The following values are obtained:

Specimen #1		Reference interval:
Sodium:	81 mmol/L	(133-145 mmol/L)
Potassium	2.0 mmol/L	(3.5-5.5 mmol/L)
Chloride	63 mmol/L	(95-100 mmol/L)
Total CO2	18 mmol/L	(21-33 mmol /L)
Glucose	1150 mg/L	(600-1000 mg/L)
BUN	140 mg/L	(50-200 mg/L)
Creatinine	15 mg/L	(7-14 mg/L)
Anion gap	3 mmol/L	(15-25 mmol/L)

QUESTION

What should the technologist’s response be to these data?

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The technologist reviews the results and recognizes that the observed electrolyte levels are not usually compatible with life. The analyses are repeated along with quality control specimens; similar results are obtained.

The technologist calls the attending physician to discuss the results and obtains the following information. This patient had no serious problems other than hypertension and had been treated with diuretics and placed on a low sodium diet. In addition, he had been drinking a lot of water (amount not stated). He was admitted to the hospital after becoming disoriented and was placed on intravenous fluids. The specimen on which the above values were obtained was clear and nonhemolyzed.

The technologist reviews the patient’s recent laboratory results in the laboratory information system (LIS). The technologist notes that the results of a renal profile ordered 5 hours earlier had totally normal results. However, a hematology profile drawn at the same time as the current renal profile had highly abnormal results, a hematocrit of 27% and a hemoglobin of 78 g/L.

The technologist calls the patient’s physician and suggests that a third sample be obtained. The physician agrees.

The following results are obtained on the third specimen:

Sodium:	139 mmol/L
Potassium	3.0 mmol/L
Chloride	106 mmol/L
Total CO2	26 mmol/L
Glucose	1180 mg/L
BUN	240 mg/L
Creatinine	25 mg/L
Calculated osmolality	294 mOsm/L	(Reference range: 280-305)
Anion gap	10 mmol/L
Measured osmolality	336 mOsm/L

QUESTION

Based on this history and the above observations about the original specimen, which of the possible reasons for the results arrived at in Question to Consider #1 are now more/less likely?

Questions to Consider

1. What kinds of instrumentation, methodology, or phlebotomy problems could yield highly abnormal results like these?
2. What might a significant difference between a measured and calculated osmolality indicate?

Answer

1. First, the specimen should be visually inspected to determine if an obvious matrix problem could be the cause of these results (see chapter 3; lipemia, hemolysis, or fibrin clots). Next, the technologist must verify the original results. If possible, another instrument or alternative methods of analysis should be employed. Running qualtity control pools will help verify that the instrument is functioning properly.
   
   While the repeat analyses are running, the technologist should check in the laboratory information system for previous results obtained on this patient to see if earlier results were similar to the current ones (Chapter 21). If the results are verified, the technologist should immediately call the attending physician since these are critical values, even if they are unlikely to truely reflect the patient's current status.
2. Upon seeing such radically different results over a short period of time, both specimens were rerun. The results were unchanged. Upon talking to the nurse, it was discovered that the patient was receiving intravenous (IV) mannitol (consisting of mannitol in a solution of water into which a minute amount of sodium bicarbonate is sometimes added as a buffer) in the same arm from which the hyponatremic specimen was drawn. The IV site was in the posterior portion of the forearm. Although the second specimen was taken from the antecubital area (which was proximal to the IV site), the nurse felt there should be no dilution of blood taken from the antecubital area. The third specimen was drawn from a hand vein (distal) to the IV site. The first specimen was drawn before the patient was given IV fluids.
   
   The problem was caused by the dilution of the venous blood sample by IV fluid before it reached the venipuncture site. This is a commonly enountered problem (see p 73) problem, one that technologists should always be concerned about. The analysis was confirmed by the much higher measured osmolality. The mannitol in the sample caused the elevation in the measured osmolality.

Answers to Questions to Consider

1. With this combination of resutls a technologist ususally thinks "preanalytical error". See chapter 3 for an extensive review of preanalytical errors, especially pages 69-73. Possible phlebotomy or laboratory problems that could have caused these results are:
   • interference from extreme lipemia (See Sodium and Potassium Method on CD-ROM).
   • fibrin clots in the sample, which can cause a “short-sampling” by an instrument’s sample probe
   • dilution of the sample by intravenous fluids that were delivered into the arm above the site used for phlebotomy.
2. The difference between the calculated and measured osmolality is striking, indicating the presence of a large amount of an unmeasured substance. This substance might have been introduced into the patient from the intravenous line.