confused about corrected anion gap...please help!!

[ketap]

hello, i am a general physician in my country..pardon me if i put this silly questions but i don't know where else to ask about this:
i want to ask: the formula for corrected anion gap : is it actual AG + [2.5 x( 4 - actual albumin)] or like the one in this website: http://fitsweb.uchc.edu/student/selectives/TimurGraham/Case_8.html : actual AG - [2.5x (4 - actual albumin) ] ?

i found several books mentioned the second when using Anion Gap to evaluate the cause of acidosis metabolic..but when using the anion gap as a prediction of a concomitant primary metabolic acidosis, i found the first one ..very confusing...


please help answer..thx u so much
warm regards,

Ketap

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[BADMD]

: is it actual AG + [2.5 x( 4 - actual albumin)

They are the same formula. This one corrects the patient's anion gap and brings it up to what it would be if the patient had a normal albumin.

normal AG - [2.5x (4 - actual albumin)

I corrected the formula for you. This one corrects down the normal anion gap to what your patient normal is for the formula.

Keep in mind that this is a rough calculation. The "normal" anion gap is pH dependent can vary based on the acidemia/alkalemia. Additionally there are lots of both unmeasured anions and cation that can change as the patient's physiology gets more out of whack. I've seen several cases where the changes offset, giving the patient a "normal" anion gap when the patient has a significant lactic acidosis.

 

[NRAI2001]

Just finished a renal elective a few weeks ago. The first equation I believe is probably what you are looking for. It corrects for a falsely low AG due to low albumin often found in liver dz and kidney dz pts.

 

[ketap]

hello..thx u so much for the replies..thx a lot
but i want to ask more if u don't mind...

1. i apologize if i am being a stubborn and bothering u, but i am still confuse : the 2nd formula i found in the book is really like this :
corrected anion gap : actual AG - {2.5 ( 4 - albumin) } ...

for ex. i saw in.Swartz surgery textbook..
but,
i noticed that when using the 2nd formula, the normal anion gap is 12 (+/- 4), and if i use the 1 st formula, the normal anion gap is reduced to only 6(+/-4) ...
my friend told me that the different range of the normal anion gap is because of different laboratory examination method being used...the AG = 12 use a method that do not acknowledge the albumin from the other anions but the new Normal AG = 6 acknowledge it..
.is it probably because of this different normal range of AG and the different method being used ,so the formula has changed or is it purely miss written by the writer?

2.i have a case : a 77 year old woman suddenly felt week since 1 day ago..she was admitted to the ER on the next day..she had a respiratory muscular weakness, seizure and also an old right brain infarct, no intracranial bleeding signs appeared, the lab results is like this:
CBC: Normal
Liver function test: AST/ALT :NORMAL
BLOOD GLUCOSE: 186 mg/dl (N: 60-180)
Ureum: 93 mg/dl ( N: 10-43 )
Creatinine: 2.3 mg/dl (N: 0.00-1.10)
Na: 130 mmol/L
K: 4 mmol/dl
CL: 97 mmol/dl
Ca total: 9.8 mg/dl
albumin : 3

blood gas analysis:
pH: 7.29 (Normal pH:7.35-7.45)
P O2: 343.0 mmHg(with NRM 15l/mnt) (N: 85-100 on room air)
PCO2 : 53.1 mmHg (N: 38-42 )
HCO3 :25.9 mmol/L (N: 19-29 )
base excess : -0.2 (N: -2.5 - +2.5)
TCO2 : 27.5 mmol/L (N: 19-25.0)

how do you approach to diagnose the ABG of this patient?

i use the 5 rules:
1. the pH is acidemia

2.because the PCO2 is high : maybe this is the respiratory acidosis

3. serum anion gap: 139 - (97+ 25.9) : 16.1
because the patient has 3.0 albumin, i correct the patient AG: 16.1 + 2,5 (4 - 3) : 18.6
(more than 10 meq/L but still below 20 meq/L) ,so it has also probably coexisting metabolic acidosis

4. check for the degree of compensation...
now i am quiet confused when stepping to this rule...because there has already 2 acid base disorder, how can i calculate the compensation with those 2 acid base disorders existing? should i count only the compensation for acidosis respiratory or should i evaluate both? how can i use it to evaluate if there is the hidden 3rd or 4th acid base disorder coexist in this patient?please give me an example ..i am confuse


thx u so much
warm regards,Ketap

 

[pgg]

Na: 130 mmol/L
K: 4 mmol/dl
CL: 97 mmol/dl
Ca total: 9.8 mg/dl
albumin : 3

blood gas analysis:
pH: 7.29 (Normal pH:7.35-7.45)
P O2: 343.0 mmHg(with NRM 15l/mnt) (N: 85-100 on room air)
PCO2 : 53.1 mmHg (N: 38-42 )
HCO3 :25.9 mmol/L (N: 19-29 )
base excess : -0.2 (N: -2.5 - +2.5)
TCO2 : 27.5 mmol/L (N: 19-25.0)

[...]

3. serum anion gap: 139 - (97+ 25.9) : 16.1

gap is130 - 97 - 26 = 7 = normal

Though the 26 from the ABG is a calculated # and ideally you'd get that from the chemistry, not the ABG.


Step 1: acidosis or alkalosis?

pH is 7.29, acidosis


Step 2: 1˚ disorder metabolic or respiratory? If there's an anion gap, the 1˚ disorder is always a gap metabolic acidosis.

There is not an anion gap. Primary disorder is respiratory acidosis (CO2 of 53).

if metabolic … is respiratory compensation appropriate?
1. metabolic acidosis → expected PaCO2 = 1.5 x HCO3- + 8 (Winter's Rule)
2. metabolic alkalosis → expected PaCO2 = 0.7 x HCO3- + 21
3. if the PaCO2 is not as predicted there is a 2˚ respiratory disorder

if respiratory … is metabolic compensation appropriate?
1. for every ↑PaCO2 by 10, acute compensation ↑HCO3- by 1, chronic compensation ↑HCO3- by 3.5
2. for every ↓PaCO2 by 10, acute compensation ↓HCO3- by 2, chronic compensation ↓HCO3- by 5
3. if the HCO3- is not as predicted, there is a 2˚ metabolic disorder (or sometimes two 2˚ metabolic disorders)

Respiratory acidosis, has CO2 elevated by about 10, so for
- acute compensation, we'd expect a HCO3- of 24 + 1 = 25
- chronic compensation, we'd expect a HCO3- of 24 + 3.5 = 27.5

Your history suggests an acute change.

HCO3- is 25.9, very close to the predicted 25, so this is an acute respiratory acidosis with no secondary metabolic disorder.

Step 3: if there is an anion gap, what are the Δgaps?
1. ΔAG = AG - 12 (this is the expected ΔHCO3-) corrected AG = 2 x albumin + .5 x PO4
2. if ΔAG ≠ ΔHCO3- there is an additional metabolic disorder
- if &#916;AG < &#916;HCO3- &#8230; ie, the HCO3- is lower than expected &#8230; there's an additional nongap metabolic acidosis
- if &#916;AG > &#916;HCO3- &#8230; ie, the HCO3- is higher than expected &#8230; there's also a metabolic alkalosis

There is no anion gap.

This is an acutely compensated respiratory acidosis.

 

[loveoforganic]

pgg, if you don't mind my asking, why does wide anion gap have to indicate a primary metabolic acidosis? It seems like if something (other than metabolic acidosis) caused hyperventilation, it would lead to wide anion gap primary respiratory alkalosis, no?

 

[NRAI2001]

pgg, if you don't mind my asking, why does wide anion gap have to indicate a primary metabolic acidosis? It seems like if something (other than metabolic acidosis) caused hyperventilation, it would lead to wide anion gap primary respiratory alkalosis, no?

I believe any AG greater than 20 must have a metabolic acidosis component to it +/- other components...

There are a few really good articles on acid base analysis that i ll try to find tommorrow.. But its not as daunting as it first appears.

 

[ketap]

gap is130 - 97 - 26 = 7 = normal

Step 2: 1&#730; disorder metabolic or respiratory? If there's an anion gap, the 1&#730; disorder is always a gap metabolic acidosis.

.

PGG: thx u for the analysis for this case..realizing that i have made an error calculations since the beggining..but, i want to ask you :

1. u have mentioned that if there is increase anion gap,the primary disorder should be the metabolic acidosis..i don't understand..shouldn't the 1st primary acid base disorder has to be the one which has the the widest changes in concentration of the BGA? as for example, if there is very huge changes increase in concentration of the PCO2 compared to the only little changes (or even still within normal ) in the HCO3, shouldn't the acidosis respiratory be the one whose acting as the 1st primary disorder (even though we have an increase anion gap)?

There is not an anion gap. Primary disorder is respiratory acidosis (CO2 of 53).

if metabolic &#8230; is respiratory compensation appropriate?
1. metabolic acidosis &#8594; expected PaCO2 = 1.5 x HCO3- + 8 (Winter's Rule)
2. metabolic alkalosis &#8594; expected PaCO2 = 0.7 x HCO3- + 21
3. if the PaCO2 is not as predicted there is a 2&#730; respiratory disorder

if respiratory &#8230; is metabolic compensation appropriate?
1. for every &#8593;PaCO2 by 10, acute compensation &#8593;HCO3- by 1, chronic compensation &#8593;HCO3- by 3.5
2. for every &#8595;PaCO2 by 10, acute compensation &#8595;HCO3- by 2, chronic compensation &#8595;HCO3- by 5
3. if the HCO3- is not as predicted, there is a 2&#730; metabolic disorder (or sometimes two 2&#730; metabolic disorders)

2.do u mean that, we need to measure the expectation only for the 1st primary disorder (even if we noticed there is 2nd primary disorder) ?

3.

&#916;AG = AG - 12 (this is the expected &#916;HCO3-) corrected AG = 2 x albumin + .5 x PO4

i don't really understand those formulas...and as i noticed, you are still using the AG = 12 as the normal...
please help, thx u so much PGG

warm regards, Ketap