I have 2 questions relating to clinical chemistry...

1st:
I have no idea where to even start with this. It seems like it should be simple, but I have no clue.

The question is...

"In serum, about 55% of total serum calcium is bound. The measurement of ionized (free) calcium with an ISE is 1.10 mM. What is an estimate of the total calcium in this specimen?"

My guess is since 55% is bound, that means 45% is free ionized species.

So... .45/1.10mM = .55/x

Solve for x and add to 1.10mM??

2nd:
I have no idea as to any reasons why this might occur...

Why is slightly higher serum sodium consistently found by evaluation by ion-selective electrode (ISE) as compared to flame photometry?

Any help would be greatly appreciated. Thank you!!

Isn't percent just (amount/total)*100 = 45. So

(1.10/total) = 0.45. Solve for total.

Thank you... Any ideas on ISE vs. flame photometry?

I was thinking that maybe flame photometry comes out consistently slightly lower because it is hard to standardize? I mean, its hard to get the exact temperature of the flame to be precise (and accurate)?

1st question:

To estimate the total calcium in the serum specimen, you need to understand that 55% of the total serum calcium is bound, and the remaining 45% is free or ionized. You are correct in thinking that you can use the ratio of bound to ionized calcium to estimate the total calcium concentration.

Let's denote the total calcium concentration as x mM. Since the free calcium concentration is 1.10 mM, you can set up the following equation:

(45% of x mM) = 1.10 mM

To solve for x, you need to convert the percentage to a decimal.

45% becomes 0.45 when expressed as a decimal.

Now, the equation becomes:

0.45x = 1.10 mM

To isolate x, divide both sides of the equation by 0.45:

x = 1.10 mM / 0.45

So, x is approximately equal to 2.444 mM.

Therefore, the estimate of the total calcium in this specimen is 2.444 mM.

2nd question:

The slightly higher serum sodium consistently found by evaluation with an ion-selective electrode (ISE) compared to flame photometry can be attributed to several factors:

1. Interference: The ISE measures sodium by detecting the electrical potential generated by the sodium ions in the sample. However, certain substances or interferences in the sample can influence the electrical potential and result in higher readings. These interferences may include proteins, lipids, or other cations present in the sample.

2. Selectivity: ISEs can be designed to be more specific to sodium ions, while flame photometry typically measures the total amount of sodium present. The selectivity of the ISE may cause it to exclude or minimize the contribution of other cations, leading to a higher reported sodium concentration.

3. Sample Preparation: The methods used to prepare the sample for analysis can also contribute to the difference in results. Flame photometry may require sample dilution or extraction steps that can affect the sodium measurement differently compared to the simpler sample preparation required for ISE.

4. Instrument Differences: Each method has its own calibration and measurement principles, which can introduce variability between instruments and laboratories. The calibration curves and settings for ISE and flame photometry may not perfectly align, resulting in slight variations in the reported sodium concentrations.

It is important to note that these reasons are general explanations, and specific factors may influence individual cases differently. To accurately determine the cause of the discrepancy, further investigation and analysis of the specific laboratory protocols and equipment involved would be required.