On appeal from his conviction of operating a motor vehicle while under the influence of alcohol, in violation of Code § 18.2-266, Allen Dale Bennett contends that the trial court erred (1) in refusing to decide whether the Commonwealth complied with Code § 18.2-268.2 and ruling that this issue was a jury question, and (2) in refusing to allow him to introduce evidence of the district court testimony of an absent witness, whom he had failed to summon. We reverse the judgment of the trial court on the first issue and remand the case.
On January 18,1997, Bennett ran a red light and almost hit a police cruiser driven by Richmond City Police Officer John B. Sheppard. Officer Sheppard followed Bennett, who stopped in an alley and exited his vehicle. Bennett appeared to be intoxicated. Officer Sheppard charged him with driving while intoxicated and transported him to the police station for a breathalyzer test. Officer Sheppard administered the test several times, but Bennett failed to produce a readable breath sample. Officer Sheppard did not offer Bennett a blood test.
Bennett moved pretrial for dismissal of the charge, contending that by failing to provide him a blood test, the Common*33wealth failed to comply with Code § 18.2-268.2 and thereby denied him potentially exculpatory evidence. See Breeden v. Commonwealth, 15 Va.App. 148, 150, 421 S.E.2d 674, 676 (1992) (decided under former Code § 18.2-268).
Code § 18.2-268.2 provides, in relevant part:
B. Any person so arrested ... shall submit to a breath test. If the breath test is unavailable or the person is physically unable to submit to the breath test, a blood test shall be given.
Id.
At the hearing on his motion, Bennett testified that he was physically unable to take the breathalyzer test due to an asthmatic condition, that he attempted unsuccessfully to blow into the breathalyzer, and that he requested a blood test, which was denied. He argued that by denying him a blood test, the Commonwealth failed to comply with Code § 18.2-268.2 and denied him potentially exculpatory evidence. Officer Sheppard testified, however, that Bennett never stated that he had an asthmatic condition, that he attempted to circumvent the breath test by blowing around the tube, and that he never requested a blood test.
The following dialogue ensued between the trial court and defense counsel:
THE COURT: Do you have anything further? Do you want to be heard?
MR. DORAN (defense counsel): Yes, sir, just briefly. Judge, under 18.2268.2 (sic), the Commonwealth is required if the breath test is unavailable or if the individual is physically unavailable to take the breath test to administer the blood test. It’s an absolute requirement.
THE COURT: Isn’t that a jury question?
MR. DORAN: No, sir, I don’t think so.
THE COURT: How am I going to say that, the, the officer says he was blowing out the side of the mouth and at one time, he wasn’t blowing at all. Then, your client comes on *34and says, well, no, I tried but it didn’t work. What am I supposed to do?
MR. DORAN: I understand that there is a question of credibility there. If you choose to resolve against the defendant, you can certainly do that.
THE COURT: Well, isn’t it a jury question? Isn’t this something that a fact finder to decide?
MR. DORAN: No, sir. It seems to me that the statute is mandatory and if there is sufficient indication, I think the standard of proof is on the probable cause. You may reject it and say that factually we have not laid the predicate for you to call into play the mandatory language of the statute. If you do that, it’s not much I can say except I appeal or otherwise.
THE COURT: Wait a minute. You are hanging your hat on the fact that he asked for a blood test?
MR. DORAN: That is right.
THE COURT: The officer said he never asked for a blood test.
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THE COURT: Isn’t this a jury question?
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THE COURT: We will have to let the jury decide. It’s not for me to decide. I can’t just do that. I’m not even going to try the case. I’m just going to sit here and watch you try the case.
The order reciting the proceedings on Bennett’s motion to dismiss states, in essential part:
Evidence and arguments of counsel having this day been presented on the defendant’s motion to dismiss these Appeals, the Court denies said motion.
Bennett contends on brief that the trial court erred in denying his motion to dismiss. His appeal was granted, however, on the question of whether the trial court erred in ruling that the Commonwealth’s compliance with Code § 18.2-268.2 was a jury issue. We limit our analysis to the question *35granted. “If this Court grants defendant’s petition for appeal on certain issues, as stated in the petition, any alteration of these issues in appellant’s opening brief, by ... changing their language or wording, ‘is contrary to accepted practice and is improper.’ ” Iglesias v. Commonwealth, 7 Va.App. 93, 96 n. 2, 372 S.E.2d 170, 171 n. 2 (1988) (en banc) (citation omitted).
“It is firmly established law in this Commonwealth that a trial court speaks only through its written orders.” Davis v. Mullins, 251 Va. 141, 148, 466 S.E.2d 90, 94 (1996). Normally, in reviewing a trial court’s factual holding, we inquire whether the record contains credible evidence supporting that holding. Were that the standard of review to be applied in this case, we would affirm the trial court’s dismissal of Bennett’s motion. Officer Sheppard’s testimony sufficiently supported that ruling.
However, upon the record presented in this case, our first inquiry is to identify the trial court’s ruling. Its holding, embodied in the order, can be read fairly only in the context of its pronouncements from the bench. From the bench, the trial court made no ruling and directed the entry of no order addressing the merits of the motion. The order itself contains no recitation suggesting a ruling on the merits. Rather, the trial court stated plainly and repeatedly that it found the issues raised by the motion inappropriate for decision by it. It refused decision on those issues and reserved them for presentation to the jury, should Bennett so elect.1 Plainly, the trial court’s dismissal of the motion was based not upon a determination on the merits, but rather upon its refusal to entertain the motion as a preliminary matter. In so ruling, the trial court erred.
The credibility issue concerning compliance with Code § 18.2-268.2 was a question of fact preliminary to a ruling of law. This question necessarily required determination by the trial court. “Issues of fact are usually left to the *36jury, but there are strong reasons here for not doing so.” 6 McCormick on Evidence § 53 (Edward W. Cleary ed., 3rd ed.1984). The motion to dismiss addressed whether the Commonwealth might prosecute the charge.
“Questions as to the competency or admissibility of testimony ... are referred to the decision of the judge. ‘As it is the province of the jury to consider what degree of credit ought to be given to evidence, so it is for the court alone to determine whether a witness is competent, or the evidence admissible. Whether there is any evidence is a question for the court; whether it is sufficient is for the jury. And whatever antecedent facts are necessary to be ascertained, for the 'purposes of deciding the question of competency— as, for example, whether a child understands the nature of an oath, or whether the confession of a prisoner was voluntary, or whether declarations offered in evidence as dying declarations were made under the immediate apprehension of death — those, and other facts of the same kind, are to be determined by the court, and not by the jury.’ ”
Mullins v. Commonwealth, 113 Va. 787, 791, 75 S.E. 193, 195-96 (1912) (citations omitted).
The action of the [trial] court in leaving evidence objected to provisionally to the jury, to be considered or rejected by them, as they might determine its admissibility or inadmissibility under the instruction given by the [trial] court, was not proper practice, as the jury has nothing to do with the admissibility of the evidence.
Id. See 7B Michie’s Jurisprudence, Evidence § 287 (1998). “The factual determinations which are necessary predicates to rulings on the admissibility of evidence and the purposes for which it is admitted [as well as related questions] are for the trial judge and not the jury.” Rabeiro v. Commonwealth, 10 Va.App. 61, 64, 389 S.E.2d 731, 732 (1990). See also C. Friend, The Law of Evidence in Virginia, §§ 1-5 (4th Ed. 1993). The same rule governs resolution of preliminary questions of fact underlying rulings of law by a trial court.
*37If the trial court believed Bennett, Code § 18.2-268.2 required dismissal of the charge. If, however, the trial court believed Officer Sheppard, the motion to dismiss should have been denied.
The trial court erred in refusing to determine the preliminary question of credibility and in refusing to rule on the merits of the motion to dismiss.
We reverse Bennett’s conviction and remand the case to the trial court for retrial if the Commonwealth be so advised. We do not address Bennett’s second assignment of error, that the trial court erred in refusing to admit evidence of the prior testimony of a witness whom the defense had failed to summon, as that issue is unlikely to occur at retrial.
Reversed and remanded.
. These issues were not presented to the jury.