Establishing a practical blood platelet threshold to avoid reporting spurious potassium results due to thrombocytosis.

BACKGROUND: Thrombocytosis is one of several pre-analytical factors which contribute to spuriously high serum potassium concentrations, yet there is little published data to guide analysts in the selection of a specific platelet count threshold above which serum potassium results become unreliable. We have studied the sensitivity and specificity of blood platelet count as a predictor of false elevations in potassium. METHODS: Paired serum and plasma potassium measurements together with full blood count were performed for 300 patients. All samples were stored at room temperature and analysed within 4 h of collection. The difference between serum and plasma potassium was plotted against blood platelet count. RESULTS: When the difference (serum-plasma) in potassium concentration was plotted against platelet count, there was a direct linear relationship. Blood platelet counts of >500 x 10(9)/L will detect elevations in serum relative to plasma potassium of >0.5 mmol/L, with a sensitivity of 71% and a specificity of 89%. CONCLUSION: It is recommended that where blood platelet count is above 500 x 10(9)/L, potassium measurements should be repeated using lithium heparin plasma. When serum potassium results of >5.4 mmol/L are obtained, it is our policy to check the platelet count if a sample is available before reporting results. If available and above 500 x 10(9)/L, potassium results are withheld and plasma requested.

Mnemonic coding system for clinical data entry into laboratory computers: its effect on quality and efficiency.

A simplified coding method for entering the clinical details found on pathology request cards was developed. The method uses a basic four letter code, derived from the initial character of the first four words in a clinical detail, being expanded to four characters with letters from the final word if the number of words is less than four. Rules were devised to cope with common medical terminology. In excess of 90% of clinical details on request cards are readily input by clerical staff using our coding system, and 8% of clinical details are used intelligently by the computer in scheduling further tests or automatically commenting on results. A carefully designed coding system such as the one outlined above could greatly facilitate input of clinical detail without the penalty of reduced throughput.

The effect of a standard oral calcium load given in two different forms on plasma ionized calcium and serum PTH.

A standard dose (1 g, 25.4 mmol) of calcium was administered both as calcium chloride in syrup and as Calcium-Sandoz Syrup (calcium glubionate and calcium galactogluconate) to 10 volunteers. Both dosage forms caused a significant rise in ionized calcium and a significant fall in concentration of serum PTH, the calcium chloride producing significantly greater changes. The urinary excretion of calcium measured over 5 h after calcium chloride was double that after Calcium-Sandoz. Calcium chloride, as expected, also gave rise to a decreased urine pH. Calcium chloride would be the better choice for an oral PTH suppression test.

Effect on 16 analytes of overnight storage of specimens collected into heparinised evacuated tubes with plasma separator.

Specimens taken into Becton Dickinson lithium heparin Vacutainers with plasma separator were compared with specimens taken into ordinary Becton Dickinson lithium heparin Vacutainers. After overnight storage, results for sodium, potassium, phosphate, albumin, lactate dehydrogenase, alkaline phosphatase, and urate concentrations showed statistical differences. Only the difference observed in urate concentration is likely to affect clinical interpretation.

An audit of dynamic function tests: errors in the timing of blood specimens.

OBJECTIVE: The interpretation of dynamic endocrine investigations is dependent on the timing of blood sampled after a stimulus. We have studied the accuracy with which doctors adhered to a prescribed protocol. DESIGN: A new request form for dynamic tests was designed to include both the planned and the actual time of blood sampling in order to allow assessment of the variance of sample timing. MEASUREMENTS: We followed 152 investigation procedures comprising 791 planned specimens. A total number of 627 timing intervals could be analysed of which 366 timing intervals did not differ from the planned interval. Protocols were divided into long and short (greater or less than 6 hours) and into those governed by clock times or 'stopwatch', i.e. minutes elapsed, intervals. RESULTS: A total of 22% specimens were taken more than 10 minutes earlier or later than planned. Six per cent of specimens were taken more than 30 minutes earlier or later than planned. CONCLUSIONS: The data presented in this study suggest that in order to improve the interpretative accuracy of dynamic tests, it would appear prudent to pay more attention to the accuracy of sample timing. New dynamic test publications should include time tolerance for the normal response.